To provide a fuel circuit of the fuel cell system, wherein a fuel pump can be scaled down, power consumption can be reduced, and required amount of fuel circulation is kept in the time of the output change in a fuel cell, including a fuel cell which generates electricity by being supplied with fuel and an oxidant; a fuel supply stream passage for sending the fuel to the fuel cell 1; a fuel circuit stream passage for making the unconsumed fuel discharged from the fuel cell merge with the fuel supply stream passage to be recirculated; the fuel pump (hydrogen pump) which takes in and sends out the unreacted fuel; and an ejector which inhales the unreacted fuel using the negative pressure generated when the fuel flows, the unreacted fuel being made to be merged with the fuel supplied to the fuel cell thereafter.