hereunder is a description of a fuel cell system according to the present invention , with reference to the drawings . fig1 is a block diagram of the fuel cell system in a first embodiment of the present invention . a fuel cell ( fc ) 1 comprises cells of solid polymer electrolyte membrane formed from a solid polymer ion exchange membrane or the like , sandwiched between an anode electrode 2 and a cathode electrode 3 , with a plurality of layers of cells each sandwiched between separators ( fig1 is a simplified diagram and therefore only a single cell is shown ). when hydrogen gas is supplied to the anode electrode 2 as fuel gas , and air including oxygen is supplied to the cathode electrode 3 as reaction gas , hydrogen ions generated by the catalytic reaction at the anode electrode 2 pass through the solid polymer electrolyte membrane and migrate to the cathode electrode 3 , giving rise to an electrochemical reaction with oxygen at the cathode electrode 3 , and generating electricity and water . air is pressurized to a predetermined pressure by a compressor 10 such as a supercharger ( s / c ) or the like , then supplied to the cathode electrode 3 of the fuel cell 1 from an air supply flow path 11 , and discharged as air off - gas from the fuel cell 1 from an air off - gas flow path 12 via a back pressure valve 13 . on the other hand , the hydrogen gas supplied from an anode gas supply system 4 having a high - pressure hydrogen tank ( h 2 ) is decreased in pressure to a predetermined pressure by a regulator 5 provided midway along a hydrogen gas supply flow path ( fuel supply flow path ) 6 , and supplied to the anode electrode 2 of the fuel cell 1 . the hydrogen gas supplied to the fuel cell 1 is employed in generation of electricity , and discharged as hydrogen off - gas to a hydrogen off - gas circulation flow path ( circulation flow path ) 7 from the fuel cell 1 . the hydrogen off - gas circulation flow path 7 is connected to the hydrogen gas supply flow path 6 downstream of the regulator 5 via an ejector 8 . thus , the hydrogen off - gas discharged from the fuel cell 1 is merged with the hydrogen gas supply flow path 6 via the ejector 8 , and thus the hydrogen off - gas is mixed with fresh hydrogen gas supplied from the anode gas supply system 4 , and supplied again to the anode electrode 2 of the fuel cell 1 . here , a branch flow path 16 is provided in the hydrogen off - gas circulation flow path 7 , and a discharge valve 15 is provided in this branch flow path 16 . furthermore , a branch flow path 18 branched from the air supply flow path 11 downstream of the compressor 10 is connected to the regulator 5 via an orifice 19 , and the regulator 5 is operated in accordance with a pilot pressure input from the branch flow path 18 . moreover , the anode gas supply system 4 , the compressor 10 , and the back pressure valve 13 are each connected to a controller ( ecu ) 14 . this controller 14 computes the electric power required for operation of the load , and sends control signals to the anode gas supply system 4 and the compressor 10 based on the computed electric power . thus , the amount of reaction gases supplied from the anode gas supply system 4 and the compressor 10 is adjusted , and the amount of electricity generated in the fuel cell 1 controlled . furthermore , the controller 14 is connected to an accelerator pedal opening sensor 21 , an atmospheric pressure sensor 22 , an airflow sensor 23 which detects the amount of airflow supplied to the cathode electrode 3 from the compressor 10 , an air pressure sensor 24 which detects the air pressure , and an intake air temperature sensor 32 , and control is conducted in accordance with the detected values detected by these sensors 21 through 24 , and 32 . hereunder is a description of this control . fig2 is a flowchart for control of the pressure of the reaction gas supplied to the fuel cell 1 , in the fuel cell system . firstly in step s 02 , a target generated current for the fuel cell 1 is computed in accordance with the accelerator pedal opening detected by the accelerator pedal opening sensor 21 . in step s 04 , the target pressure is set so that maximum efficiency is reached at the target generated current . this target pressure is set using fig6 . fig6 is a graph showing the relationship between target generated current and target pressure in the standard state ( state wherein the atmospheric pressure value is the reference pressure value , and intake air temperature value is the reference temperature value ). here , the inlet pressure of the cathode electrode 3 is set as the standard pressure , however , the outlet pressure may also be set . in step s 06 , the atmospheric pressure po is detected by the atmospheric pressure sensor 22 , and a correction coefficient is computed in accordance with this atmospheric pressure po . this correction coefficient is computed using fig7 . fig7 is a graph showing the relationship between atmospheric pressure and the correction coefficient . in the present embodiment , the pressure value at a predetermined height above sea level ( for example , 0 m above sea level ) is set as the reference pressure value , and the correction coefficient at this reference value is set to 1 . 0 . if the atmospheric pressure detected by the atmospheric pressure sensor 22 is greater than the reference pressure value , the correction coefficient is increased , and if the atmospheric pressure detected by the atmospheric pressure sensor 22 is less than the reference pressure value , the correction coefficient is decreased . in step s 08 , the target pressure is corrected in accordance with the correction coefficient . this correction is conducted by multiplying the target pressure value set in step s 04 by the correction coefficient . in step s 10 , the inlet pressure ( actual pressure ) of the cathode electrode 3 is detected by the atmospheric pressure sensor 24 . in step s 12 , the opening of the back pressure valve 13 is feedback - controlled so that the target pressure corrected in step s 08 matches the actual pressure , and the processing for this flowchart is completed . fig3 is a flowchart for control of the flow of reaction gas supplied to the fuel cell 1 . in step s 22 , the target generated current for the fuel cell 1 is computed in accordance with the accelerator opening detected by the accelerator pedal opening sensor 21 . in step s 04 the target airflow is set in accordance with the target generated current . here , the inlet flow of the cathode electrode 3 is set as the target airflow , however the outlet flow may also be set . in step s 26 , the inlet flow ( actual flow ) of the cathode electrode 3 is detected by the airflow sensor 23 . in step s 28 , the rotating speed of the compressor 10 is feedback - controlled so that the actual flow matches the target airflow , and the processing for this flowchart is completed . in this way , controlling the airflow for the cathode electrode 3 by the rotating speed of the compressor 10 , and on the other hand independently controlling the air pressure of the cathode electrode 3 by the back pressure valve 13 , is desirable from the point of enabling accurate control of the respective amounts . the compressor 10 and the back pressure valve 13 can be used together for control of the airflow and the air pressure . fig5 is a graph showing the relationship between the cathode gas pressure and electric power generated by the fuel cell 1 , and the electric power consumption of the compressor . in this figure , line l 1 represents the electric power generated by the fuel cell 1 , line l 2 represents the electric power consumption of the compressor 10 at the target pressure without the correction shown in steps s 06 and s 08 , and line l 3 represents the electric power consumption of the compressor 10 at the corrected target pressure . moreover , pa_c is a pressure value approximately the same as the reference pressure value , and pa_a ( equal to pa_b ) is a pressure value a certain amount less than the reference pressure value . as shown in the same figure , when the atmospheric pressure is comparatively close to the reference pressure value ( pa_c ), the electric power consumption of the compressor 10 is approximately equal irrespective of whether or not correction is used , and the external output becomes approximately equal . however , when the atmospheric pressure is less than the reference pressure value ( pa_a , pa_b ), unless the target pressure is corrected , the electric power consumption of the compressor 10 increases , and the electric power extracted from the fuel cell 1 minus the compressor 10 consumption decreases significantly . conversely , when the target pressure is corrected , the increase in electric power consumption of the compressor 10 can be suppressed , and the decrease in external output can be suppressed . furthermore , the difference between effects of the fuel cell system of the present embodiment and the conventional fuel cell system is explained using fig9 and fig1 . as shown in fig1 , in the conventional fuel cell system , since the electric power consumption of the compressor 10 is low at a low height above sea level ( height above sea level : low ), an external output ( load output ) comparatively similar to that required can be obtained . however , even when height above sea level increases to a certain extent ( height above sea level : medium ), the fuel cell system attempts to maintain the inlet pressure of the cathode electrode 3 at a high state similar to that at a low height above sea level , and the generated electric power output therefore remains approximately unchanged . however , since the amount of compressor 10 work increases due to the decrease in pressure , the electric power consumption of the compressor 10 increases . therefore the external output decreases significantly as a result . when the height above sea level increases further ( height above sea level : high ), the performance limit of the compressor 10 is reached and the generated electric power decreases , while on the other hand , the electric power consumption of the compressor 10 increases further , so that the external output decreases further . conversely , with the fuel cell system of the present embodiment , since the electric power consumption of the compressor 10 is low at a low height above sea level ( height above sea level : low ), a high external output can be obtained . moreover , even when the height above sea level increases to a certain extent ( height above sea level : medium ), correction is applied to decrease the cathode electrode 3 inlet pressure , and the generated electric power decreases , while the increase in the amount of work of the compressor 10 can be suppressed . therefore , the overall electricity generation efficiency can be maintained in a high state similar to that wherein the height above sea level is low . furthermore , when the height above sea level increases further ( height above sea level : high ), even if the performance limit of the compressor 10 is reached , since correction is performed to further decrease the inlet pressure of the cathode electrode 3 , the increase in the amount of work of the compressor 10 can be suppressed , and electricity generation efficiency can be increased beyond the conventional case . next , the fuel cell system in a second embodiment of the present invention is described using fig1 . the present embodiment differs from the first embodiment in the use of an intake air temperature sensor 32 in place of the atmospheric pressure sensor 22 , and control is in accordance with the intake air temperature ta . this is described using fig4 . fig4 is a flowchart for pressure control processing in the fuel cell system of fig1 . as shown in this figure , in this flowchart , in place of the processing in step s 06 in fig2 , the intake air temperature ta is detected by the intake air temperature sensor 32 and the correction coefficient is computed in accordance with the intake air temperature ta . this correction coefficient is computed using fig8 . fig8 is a graph showing the relationship between intake air temperature and correction coefficient . in the present embodiment , the intake air temperature value at a predetermined height above sea level ( for example , 0 m above sea level ) is set as the reference temperature value , and the correction coefficient at this reference temperature value is set to 1 . 0 . then when the intake air temperature detected by the intake air temperature sensor 32 is greater than the reference temperature value , the correction coefficient is decreased , and when the intake air temperature detected by the intake air temperature sensor 32 is less than the reference temperature value , the correction coefficient is increased . since the volume of air in the compressor 10 also increases when the intake air temperature is greater than the reference temperature value , then when the fuel cell system attempts to maintain the pressure of the cathode electrode 3 to that for when the intake pressure is the same as the reference value , the electric power consumption of the compressor 10 increases . therefore , by correcting as in step s 32 , the increase in electric power consumption of the compressor 10 can be suppressed . in this manner , in the present embodiment , even if the intake air temperature varies , the electric power consumption of the compressor 10 can be suppressed , and a decrease in the external output can be prevented , and electric power can therefore be generated efficiently even if the ambient temperature environment varies . according to the present invention , even when the atmospheric pressure varies , the electric power consumption of the compressor can be suppressed , and a decrease in external output can be prevented . therefore efficient electric power generation can be performed even if the ambient pressure environment varies . according to the present invention , efficient electric power generation can be performed even if the atmospheric pressure decreases . according to the present invention , even when the intake air temperature varies , the electric power consumption of the compressor can be suppressed , and a decrease in external output can be prevented . therefore efficient electric power generation can be performed even if the ambient temperature environment varies . according to the present invention , efficient electric power generation can be performed even if the intake air temperature increases . the details of the present invention are naturally not limited to the embodiments . for example , in the embodiments , a fuel cell system as mounted in a vehicle has been explained , however , this is not limited to a vehicle . moreover , in the embodiments , the reference pressure value and the reference temperature value are set , and if the detected atmospheric pressure is less than the reference pressure value , or the detected intake air temperature is greater than the reference temperature value , the cathode pressure target value is controlled to decrease . however , the reference pressure value and the reference temperature value need not be set . that is to say , when the detected atmospheric pressure target value tends to decrease , or when the detected intake air temperature tends to increase , the cathode pressure target value may be controlled to decrease . furthermore , the control in the first embodiment and the control in the second embodiment may be used together . while preferred embodiments of the invention have been described and illustrated above , it should be understood that these are exemplary of the invention and are not to be considered as limiting . additions , omissions , substitutions , and other modifications can be made without departing from the spirit or scope of the present invention . accordingly , the invention is not to be considered as being limited by the foregoing description , and is only limited by the scope of the appended claims .