To provide a fuel battery power generation system and a gas pressure reducing valve for the fuel cell power generation system which is advantageous for adjusting the characteristics of the gas flow rate/pressure of gas containing active substances while increasing responsiveness.

This fuel cell power generation system is provided with a gas pressure reducing valve 1, and the gas pressure reducing valve 1 is provided with a body 2 having a diaphragm chamber 20, a deformable diaphragm 3 for dividing the diaphragm chamber 20 into a first chamber 21 and a second chamber 22, a primary side passage 6 to which gas containing the active materials is supplied, a secondary side passage 71 connected through the first chamber 21 to the fuel cell, a diaphragm opening degree varying mechanism 4 installed between the primary side passage 6 and the secondary side passage 71 for supplying gas through the first chamber 21 to the secondary side passage 71 by reducing the gas flow rate of the primary side passage 6, an elastic member 7 for energizing the diaphragm 3 to a direction where the diaphragm opening degree of the diaphragm opening degree varying mechanism 4 is increased or decreased and an actuator 9 for adjusting the energizing force of the elastic member 7.