an embodiment of the display device of a construction machine according to the invention will be described with reference to the accompanying drawings . as shown in fig2 , a monitor panel 10 of this embodiment is disposed in a cabin 40 of a construction machine . on the other hand , the cabin 40 is provided therein with an illumination switch 45 , as shown in fig3 . where the indicator 45 a of the illumination switch 45 indicates a first switch position 46 , all the floodlight of the display device , the headlight and the working light of the construction machine are off . where the indicator 45 a of the illumination switch 45 indicates a second switch position 47 , the floodlight of the display device is on to illuminate the display device . in this case , on the other hand , the headlight and the working light of the construction machine are off . where the indicator 45 a of the illumination switch 45 indicates a third switch position 48 , not only the floodlight of the display device but also the headlight and the working light of the construction machine are turned on . fig1 shows the exterior of the display device 10 of the embodiment . the display device 10 of the embodiment has a display screen 30 and a control unit 20 integrated , and is called the “ monitor panel 10 ”. fig1 a shows the exterior of the monitor panel 10 at the daytime , and fig1 b shows the exterior of the monitor panel 10 at the nighttime . the display screen 30 is composed of a background portion 31 , bar graphs 32 and caution marks 33 . in the background portion 31 , more specifically , the bar graphs 32 and the caution marks 33 are arranged as the display portions . here , the bar graphs 32 are display portions for indicating , when turned on , the numerical values of the water temperature and the fuel residual . on the other hand , the caution marks 33 are display portions for indicating , when turned on , the abnormal conditions of the water temperature , the fuel residual , the oil temperature , the oil pressure and the battery voltage . the background portion 31 , the bar graphs 32 and the caution marks 33 are lit in different display colors . here , the “ caution marks ” imply the aforementioned abnormal displays in a narrow sense . however , the “ caution marks ” to be used hereinafter are deemed to have a wide sense containing the parking brake indication , the pilot indication such as the preheat indication , and the message indication ( in letters ). in the monitor panel 10 of fig1 , the background portion 31 occupies one half or more of the area of the display screen 30 . in other words , in the display screen 30 , the area occupied by the background portion 31 is larger than that occupied by the bar graphs 32 and the caution marks 33 . for the display screen 30 , there can be used the dot - matrix lcd ( or dot - matrix type liquid crystal display ), the el ( or electro - luminescence ) or the led ( or light emitting diode ). this embodiment imagines the liquid crystal display . by controlling the liquid crystal display device , therefore , the background portion 31 , the bar graphs 32 and the caution marks 33 on the display screen 30 are discriminated and displayed in display colors of different hues ( e . g ., light blue , green and red ). this embodiment is characterized , as will be described hereinafter , in that the display color of the background portion 31 is displayed in a brightness different from those of the bar graphs 32 and the caution marks 33 to change the brightness difference ( or contrast ) between the background portion 31 , and the bar graphs 32 and the caution marks 33 . the “ brightness ” used here implies the brightness as one of the three primaries of colors . in this embodiment , there is imagined the color liquid crystal display screen 30 which can display the background portion 31 , the bar graphs 32 and the caution marks 33 in the chromatic colors ( i . e ., the light blue , green and red colors ) however , these background portion 31 , bar graphs 32 and caution marks 33 may be displayed in the achromatic colors ( i . e ., the white , grey and black colors ). the monitor panel 10 is provided on its control unit 20 with a plurality of select switches 21 , a plurality of indication lamps 22 and a brightness changing switch 23 . the select switches 21 output signals of different contents according to the number of depressions to the control device of the construction machine . when the select switch 21 for the working mode is selectively controlled to the “ heavy excavate ”, for example , a signal commanding the heavy excavation is outputted to the control device so that the speed of the engine and the discharge of the hydraulic pump are controlled to fit the heavy excavation . the indication lamp 22 indicates the content , as selected by the select switch 21 , when turned on . when the select switch 21 for the working mode is selectively controlled to the “ heavy excavate ”, for example , the indication lamp 22 corresponding to the “ heavy excavate ” is turned on . the brightness changing switch 23 changes the brightness of the background portion 31 of the display screen 30 at two stages . each time the brightness changing switch 23 is pushed , there are alternately selected a higher brightness and a lower brightness . when the brightness changing switch 23 is pushed for the higher brightness of the background portion 31 , a signal instructing the lower brightness is outputted to the liquid crystal drive device . as a result , the brightness of the display color of the background portion 31 is changed to the lower one . for example , the background portion 31 is changed from the “ thin light blue ” to the “ thick light blue ”. when the brightness changing switch 23 is pushed for the lower brightness of the background portion 31 , on the other hand , a signal instructing the higher brightness is outputted to the liquid crystal drive device . as a result , the brightness of the display color of the background portion 31 is changed to the higher one . for example , the background portion 31 is changed from the “ thick light blue ” to the “ thin light blue ”. when the brightness of the background portion 31 is thus changed , its difference is changed between the background portion 31 , and the bar graphs 32 and the caution marks 33 . here will be described the actions of the monitor panel 10 of fig1 . it is now assumed that the display color of the background portion 31 of the display screen 30 is the “ thin light blue ”, as shown in fig1 a . when the brightness of the outside of the cabin 40 drops at the nighttime , there arises a difference between the brightness of the outside of the cabin 40 and the brightness of the inside of the cabin 40 . as a result , the background portion 31 occupying most of the display area of the display screen 30 has a high brightness of the “ thin light blue ” so that the display screen 30 is easily reflected on the glass shields around the cabin 40 by the light emitted from the display screen 30 . therefore , the operator pushes the brightness changing switch 23 so as to reduce the reflection on the glass shields . as a result , the signal instructing the lower brightness is outputted to the liquid crystal drive device so that the brightness of the display color of the background portion 31 is changed to the “ thick light blue ” of the lower brightness , as shown in fig1 b . as a result , the reflection of the display screen 30 on the glass shields can be substantially eliminated . moreover , the brightness of the display screen 30 is not reduced so that the visibility of the display screen 30 is not degraded . therefore , the operator can retain the field of view at the nighttime to drive the construction machine safely . if the brightness of the outside of the cabin 40 rises at the daytime but the display color of the background portion 31 on the display screen 30 remains in the low brightness of the “ thick light blue ”, on the other hand , the display color of the background portion 31 and the display colors of the bar graphs 32 and the caution marks 33 on the display screen 30 become visibly poor in the brightness difference ( or contrast ) so that the bar graphs 32 and the caution marks 33 on the display screen 30 are hard to observe . therefore , the operator pushes the brightness changing switch 23 so that the bar graphs 32 and the caution marks 33 on the display screen 30 may be easily observed . as a result , the signal instructing the higher brightness is outputted to the liquid crystal display device to change the brightness of the display color of the background portion 31 into the “ thin light blue ” of the higher brightness , as shown in fig1 a . as a result , the brightness difference ( or contrast ) between the display color of the background portion 31 and the display color of the bar graphs 32 and the caution marks 33 on the display screen 30 becomes satisfactory in the visibility so that the bar graphs 32 and the caution marks 33 on the display screen 30 can be easily observed . here in this embodiment , the display color of the background portion 31 is changed by pushing the brightness changing switch 23 . however , the brightness of the background portion 31 on the display screen 30 may be changed not by providing the brightness changing switch 23 but in association with the illumination switch 45 shown in fig3 . at the nighttime , more specifically , when the indicator 45 a of the illumination switch 45 is controlled from the first switch position 46 to the second switch position 47 , the floodlight of the monitor panel 10 is turned on to illuminate the monitor panel 10 . when the indicator 45 a of the illumination switch 45 is controlled to indicate the third switch position 48 , on the other hand , the headlight and the working light of the construction machine are turned on along with the illumination of the monitor panel 10 . associatively as the indicator 45 a of the illumination switch 45 is controlled to indicate the second switch position 47 from the first switch position 46 , the signal instructing the lower brightness is outputted to the liquid crystal display device so that the brightness of the display color of the background portion 31 is changed to the “ thick light blue ” of the lower brightness , as shown in fig1 b . here , the brightness may be changed associatively as the indicator 45 a of the illumination switch 45 is controlled to indicate the third switch position 48 . when the indicator 45 a of the illumination switch 45 is controlled at the daytime to indicate the first switch position 46 from the second or third switch position 47 or 48 , the illumination switch is turned off so that the monitor panel 10 of the cabin 40 is not illuminated . associatively with this , the signal to instruct the higher brightness is outputted to the liquid crystal display device so that the brightness of the display color of the background portion 31 is changed to the “ thin light blue ” of the higher brightness . on the other hand , the brightness of the display color of the background portion 31 may be changed not manually but automatically . for example , the cockpit 40 of the construction machine is provided inside or outside thereof with a brightness sensor . this brightness sensor can be exemplified by an optical sensor . when the illuminance detected by the illumination sensor becomes lower in this case than a predetermined threshold value , the signal instructing the lower brightness is outputted to the liquid crystal display device . when the illuminance detected by the illumination sensor exceeds a predetermined threshold value , on the other hand , the signal instructing the higher brightness is outputted to the liquid crystal display device . at the nighttime when the illuminance detected by the illumination sensor is lower than the predetermined threshold value , therefore , the signal instructing the lower brightness is outputted to the liquid crystal display device so that the brightness of the display color of the background portion 31 is changed to the “ thick light blue ” of the lower brightness , as shown in fig1 b . at the daytime when the illuminance detected by the illumination sensor is higher than the predetermined threshold value , on the other hand , the signal instructing the higher brightness is outputted to the liquid crystal display device so that the brightness of the display color of the background portion 31 is changed to the “ thin light blue ” of the higher brightness , as shown in fig1 a . in the embodiment thus far described , the brightness is changed according to the illuminance of the outside of the cabin 40 . however , the brightness may be changed responsively as the construction machine becomes abnormal . when it is detected that the construction machine is abnormal , the caution mark 33 corresponding to the abnormal content is lit . associatively as this caution mark is lit , moreover , the signal to instruct the lower brightness is outputted to the liquid crystal display device . where no abnormality is detected in the construction machine , the signal to instruct the higher brightness is outputted to the liquid crystal display device . when it is detected that the construction machine is abnormal , therefore , the caution mark 33 corresponding to the abnormal content is lit . in response to this lighting , the signal to instruct the lower brightness is outputted to the liquid crystal display device so that the brightness of the display color of the background portion 31 is changed to the “ thick light blue ” of the lower brightness , as shown in fig1 b . where it is detected that the construction machine is not abnormal , on the other hand , the signal to instruct the higher brightness is outputted to the liquid crystal display device so that the brightness of the display color of the background portion 31 is changed to the “ thin light blue ” of the higher brightness , as shown in fig1 a . thus , the brightness of the display color of the background portion 31 of the display screen 30 is changed from the brightness different from that at the time when the abnormality is not detected . at an abnormal time , according to this embodiment , the display color of the background portion 31 occupying most of the display area of the display screen 30 changes in the brightness . as compared with the case in which only the caution mark 33 occupying the small area of the display screen 30 is lit , therefore , the operator can recognize the abnormality quickly . as a result , a quick action can be taken on the abnormality of the construction machine while exerting no adverse affect on the construction machine . if the difference between the brightness of the caution mark 33 , as lit , and the brightness of the surrounding background portion 31 is made so as to clarify the caution mark 33 , on the other hand , the operator can recognize the abnormality more quickly from the screen 30 . if a different brightness is made to correspond to the kinds of abnormalities , on the other hand , the abnormal content can be discriminated according to the brightness . where the abnormality is informed by changing the brightness of the background portion 31 , it is possible to omit the arrangement of the caution marks 33 . fig4 is a diagram showing the monitor panel 10 which is provided with a display screen 50 according to a second embodiment . the description of the same components as those of the monitor panel of fig1 will be properly omitted by designating them by the common reference numerals . the monitor panel 10 shown in fig1 can change the brightnesses of the bar graphs 32 and the caution marks 33 of the display screen 50 . the display screen 50 is composed of the background portion 31 , the bar graphs 32 and the caution marks 33 . most of the display area of the display screen 50 is occupied by the bar graphs 32 and the caution marks 33 . the display screen 50 has a function to display a plurality of colors and the brightness of the screen . for the display screen 50 , there can be used the dot - matrix lcd ( or dot - matrix type liquid crystal display ), the el ( or electro - luminescence ) or the led ( or light emitting diode ). in the display screen 50 , the display area of the bar graphs 32 and the caution marks 33 is larger than that of the background portion 31 so that the most of the display area of the display screen 50 to be reflected on the glass shields is occupied by the bar graphs 32 and the caution marks 33 . at the second embodiment , therefore , there are changed the brightnesses of the bar graphs 32 and the caution marks 33 . in this case , the brightness of the bar graphs 32 can also be exclusively changed . where the caution marks 33 occupy most of the display area of the display screen 30 , the brightness of only the caution marks 33 may be arbitrarily changed . irrespective of the sizes of the display area of the background portion 31 and the display areas of the bar graphs 32 and the caution marks 33 , on the other hand , the brightness of the display color of the background portion 31 may be arbitrarily changed . this is because the bar graphs 32 and the caution marks 33 themselves may become hard to observe if the brightnesses of the display colors of the bar graphs 32 and the caution marks 33 are changed . in the case of many display colors of the bar graphs 32 and the caution marks 33 , on the other hand , the control is complicated for changing the brightnesses of the display colors . by changing the brightness of the background portion 31 having a small number of display colors , the controls are facilitated for changing the brightnesses of the display colors . in the embodiments thus far described , on the other hand , any of the background portion 31 , the bar graphs 32 and the caution marks 33 of the display screen 30 is changed in brightness . in the invention , however , it is not prohibited to change the brightness of the display screen 30 , i . e ., the brightnesses of both the background portion 31 , and the bar graphs 32 and the caution marks 33 . on the other hand , the foregoing embodiments have been described assuming that the brightness of the display color of the background portion 31 or the bar graphs 32 and the caution marks 33 is changed at the two stages . however , the brightness of the display color may be freely adjusted by the operator . on the other hand , the foregoing embodiments have been described assuming that what is displayed on the display screen 30 is the background portion 31 , the bar graphs 32 and the caution marks 33 . however , the display on the display screen 30 should not be limited to the background portion 31 , the bar graphs 32 and the caution marks 33 but can be applied to any item if this item can be displayed . here , the foregoing embodiments have been described assuming that the brightness is changed of the three primaries of colors . by changing the saturation or hue of the display color , however , it is also possible to change the difference in the saturation or the difference in the hue between the background portion 31 , and the bar graphs 32 and the caution marks 33 .