preferred aspects and embodiments of the present invention will now be described with reference to the accompanying drawings . incidentally , all drawings merely illustrate rough forms , sizes and positional relations of elements and members to understand the present invention . an explanation will be given of a case that there is a contamination on transmission area in a rear face of an exposure mask in the first embodiment . fig1 ( a ) though 2 ( b ) are used to explain the first embodiment . fig3 is a plan view illustrating roughly a part of an exposure mask . in an exposure mask 11 , at least two areas 15 , each of which has a specific shade pattern 13 are arranged ( see fig3 ). one area is called a first area 15a , and another area is selected among areas expect for the first area 15a and called a second area 15b . both the first area 15a and the second area 15b are respectively provided with similar shade patterns 13 and similar transmission areas 14 , and a size and a form of the first area 15a are similar to those of the second area 15b . now , the contamination inspection is carried out on the rear face of the mask 11 as to the first area 15a and the second area 15b at the same time . the inspection is carried out by moving the exposure mask 11 along a line x -- x . the inspection light falls from an irradiation start point x 1 to an irradiation end point x 2 in the first area 15a and from an irradiation start point x 3 to an irradiation end point x 4 in the second area 15b on the line x -- x , and the exposure mask 11 is sequentially moved along a direction of y -- y so as to inspect all inspection areas . here , inspection data obtained by once applying the inspection light to a portion on which there is a contamination are extracted . fig1 ( a ) is a sectional view illustrating the exposure mask 11 along the line x -- x in fig3 . first , a first inspection light 19 is applied from a light source 17 through a lens b to the front face of the first area 15a . at the same time , a second inspection light 23 is applied through a lens d to the rear face of the second area 15b ( see fig1 ( a )). the lenses a - d are optical lenses which magnifications are lowered suitably not so as to detect a small contamination which has no actual influence on exposure to the front face of the mask 11 and to the rear face of the mask 11 ( 100 optical magnification in this embodiment ). the lenses a - d are set at the same magnification . a lens which is usually used to inspect whether there is a contamination or not is set at 1000 - 2000 magnification , however , in this embodiment , since the object is to detect only a contamination which has an actual influence on exposure to the rear face of the mask 11 , the magnification of the lens is lowered . fig1 ( b ) is a characteristic view illustrating electric signals obtained by light receiving parts 25 , 29 and monitored by an indicator 21 in the inspection system . a horizontal direction indicates a irradiation position , and a longitudinal direction indicates electric signal levels corresponding to the luminous intensity of the detected light . the electric signal level becomes higher , as the luminous intensity becomes stronger . in fig1 ( b ), the detected result in the first area 15a and that in the second area 15b are overlapped . a irradiation distance between the irradiation start point x 1 and an irradiation end point x 2 in the first area 15a is similar to that between an irradiation start point x 3 and an irradiation end point x 4 in the second area 15b . a point at a distance l away from the irradiation start point x 1 to the irradiation end point x 2 in the first area 15a and a point at a distance l away from the irradiation start point x 3 to the irradiation end point x 4 in the second area 15b are respectively set as x m and x n . in fig1 ( b ), the x m is just over x n . a first electric signal ( 1 ) indicates an electric signal to be a detected result in the first area 15a , and a second electric signal ( 2 ) indicates an electric signal to be a detected result in the second area 15b . then , a third electric signal ( 3 ) indicates an electric signal composed of the detected result in the first area 15a and that in the second area 15b . when the first inspection light 19 falls on the front face of the first area 15a from the irradiation start point x 1 to the irradiation end point x 2 , the light 19 passes through the transmission area 14 and reaches a first light receiving part 25 provided at the rear side of the mask 11 , whereby an electric signal corresponding to the luminous intensity is detected . this electric signal is the first electric signal ( 1 ). as to the first electric signal ( 1 ), the light reflected by the shade pattern 13 does not reach the first light receiving part 25 , therefore , the electric signal level becomes low . moreover , the light which reaches a contamination 27 on the rear face of the transmission area 14 in the first area 15a is scattered by irregularities of the contamination 27 , therefore , little light reaches the first light receiving part 25 . thus , the electric signal level drops rapidly at the portion of the contamination 27 ( see fig1 ( b )). the second inspection light 23 falls on the rear face in the second area 15b from the irradiation start point x 3 to the irradiation end point x 4 , whereby the second electric signal ( 2 ) is obtained . as to the second electric signal ( 2 ), the light 23 passes through the transmission area 14 to the front face , and no light 23 reaches a second light receiving part 29 provided at the rear side of the mask 11 , therefore , the electric signal becomes a low level . the light reflected by the shade pattern 13 reaches the second light receiving part 29 , therefore , the electric signal of a high level is detected ( see fig1 ( b )). the first electric signal ( 1 ) and the second electric signal ( 2 ) are composed , whereby a first composite electric signal ( 3 ) is obtained . the first electric signal ( 1 ) is a signal of a transmitting light detected by applying the first inspection light to the front face of the first area 15a . the second electric signal ( 2 ) is a signal of a reflected light detected by applying the second inspection light to the rear face of the second area 15b . the first composite electric signal ( 3 ) is a flat signal which there is no difference between the transmission area 14 and the shade pattern 13 , since a high level portion of the electric signal in the first area 15a corresponds to a high level portion of the electric signal in the second area 15b and both portions negate each other ( see fig1 ( b )). then , there is very few cases that contaminations of the same forms and the same sizes exist at the same positions in both the first area 15a and the second area 15b , therefore , there is a variation in the signal at the position which the contamination 27 exists when both signals are composite . thus , the first electric signal ( 1 ) and the second electric signal ( 2 ) are composite , whereby it is possible to obtain the first composite electric signal ( 3 ) indicating only the contamination 27 . next , a third electric signal ( 4 ) and the fourth electric signal ( 5 ) are obtained . the third electric signal ( 4 ) is a signal of the reflected light detected by applying a third inspection light 31 to the rear face of the first area 15a . the fourth electric signal ( 5 ) is a signal of the transmitting light detected by applying a fourth inspection light 33 to the front face of the second area 15b . the inspection is carried out by the same procedure to obtain signals of the transmitting light in the first area 15a and the reflected light in the second area 15b ( see fig2 ( a )). the inspection lights can be selectively applied to the front face of the mask 11 and to the rear face by mirrors or the like ( the reverse is also possible ) simply . as to the third electric signal ( 4 ), since the contamination 27 exists at the transmission area 14 in the first area 15a and the electric signal becomes a low level around the contamination 27 in the first area 15a , the contamination 27 stands out not much . thus , the variation indicating the contamination 27 in the second composite electric signal ( 6 ) becomes weak ( see fig2 ( b )). thus , it is desired to detect the contamination 27 by comparing and examining the first composite electric signal ( 3 ) which is the detected result obtained from the transmitting light in the first area 15a and the reflected light in the second area 15b ( see fig1 ( b )) and the second composite electric signal ( 6 ) which is the detected result obtained from the reflected light in the first area 15a and the transmitting light in the second area 15b ( see fig2 ( b )). as a result , inspection lights are passed through the lenses a - d set at low magnification ( about 100 optical magnification in this embodiment ), whereby it is possible to automatically detect only a contamination which has an actual influence on exposure , and it is possible to detect only a contamination easily though the shade pattern 13 is detailed and complex . further , a light obtained from the same light source such as a mercury lamp of the same wave length ( such as 600 nm ) is used for the first , second , third and fourth inspection lights . an explanation will be given of a contamination inspection method according to the second embodiment in a case that a contamination exists at the opposite side of the shade pattern 13 not at the transmission area 14 with reference to fig4 ( a ) through 4 ( c ). forms and sizes of the shade pattern 13 and areas are similar to those in the first embodiment . similarly to the first embodiment , a first electric signal ( 7 ) and a second electric signal ( 8 ) are composite to obtain a first composite electric signal ( 9 ) ( see fig4 ( b )), and then a third electric signal ( 10 ) and a fourth electric signal ( 11 ) are composite to obtain a second composite electric signal ( 12 ) ( see fig4 ( c )). the first composite electric signal ( 8 ) is a detected result obtained from the transmitting light in the first area 15a and the reflected light in the second area 15b . the second composite electric signal ( 12 ) is a detected result obtained from the reflected light in the first area 15a and the transmitting light in the second area 15b . compared the first composite electric signal ( 9 ) and the second composite electric signal ( 12 ), the first composite electric signal ( 9 ) shows apparently that a contamination 27a exists at the opposite side of the shade pattern , on the contrary , the second composite electric signal ( 12 ) does not show the contamination 27a since the fourth inspection light 33 is blocked by the shade pattern 13 in the second area 15b . there is no case that the contamination 27a directly influences on exposure when the contamination 27a is fixed to the opposite side of the shade pattern 13 . it , however , is necessary to detect the contamination 27a since there is a case that the contamination 27a moves to the transmission area 14 with shocks by conveying the mask 11 and so on . thus , in this case , the inspection requires to compare the first composite electric signal ( 9 ) ( see fig4 ( b )) with the second composite electric signal ( 12 ) ( see fig4 ( c )). the first composite electric signal ( 9 ) is the detected result obtained from the transmitting light in the first area 15a and the reflected light in the second area 15b . the second composite electric signal ( 12 ) is the detected result obtained from the reflected light in the first area 15a and the transmitting light in the second area 15b . thus , it is possible to automatically detect only the contamination 27a which there is a possibility of influence on actual exposure among contaminations on the rear face of the exposure mask 11 . as above described , according to the present invention , for the contamination inspection on an exposure mask with at least two areas , each of which has a specific shade pattern , a composite electric signal is obtained by composing the electric signal of the transmitting light detected by applying the inspection light to the front face in one area of the exposure mask and the electric signal of the reflected light detected by applying the inspection light to the rear face in another area of the exposure mask . then , another composite electric signal is obtained by composing the electric signal of the reflected light detected by applying the inspection light to the rear face in one area of the exposure mask and the electric signal of the transmitting light detected by irradiating the inspection light to the front face in another area of the exposure mask . then , since variations in these two composite electric signals represent contaminations on the whole rear face of the mask , it is possible to detect contaminations on the rear face of the mask with detailed and complex shade patterns easily . moreover , two areas can be inspected at the same time , therefore , it is possible to further shorten the inspection time as the number of areas to be inspected becomes larger . further , the inspection lights are passed through optical lenses which magnifications are lowered suitably to detect only a contamination which there is a possibility of an actual influence on exposure , and the inspection lights are applied to the mask , therefore , there is no case to detect a contamination which is unnecessary to be inspected . thus , it is possible to shorten a time to determine whether a contamination influences on exposure or not , and it is possible to supply an exposure mask of a high quality . there is a possibility that a contamination on the opposite side of the shade pattern of the mask moves into the transmission area and influences on exposure with shocks by conveying the mask and so on , however , according to the contamination inspection method of the present invention , it is possible to detect all contaminations at the opposite side of the shade pattern , therefore , there is no possibility that a contamination influences on exposure . thus , it is possible to shorten a time to detect a contamination and to determine whether a contamination influences on exposure or not , and it is possible to supply an exposure mask of a high quality . additionally , the explanation is given of the case that there is a contamination on a mask in these embodiment , however , it is also possible to detect a flaw , a defect and the like of the mask . further , the explanation is given of a positive photo mask in these embodiment , however , it is also possible to apply the present invention to a negative photo mask . this invention being thus described , it will be obvious that same may be varied in various ways . saturations are not to be regarded as departure from the spirit and scope of the invention , and all such modifications would be obvious for one skilled in the art intended to be included within the scope of the following claims .