FUEL CELL STACK COMBINING METHOD

A fuel cell system includes fuel cell modules connected in parallel and each including fuel cell stacks connected in series. A tester includes: an output power acquirer that acquires an output power value for each fuel cell stack; a deterioration estimator that estimates a degree of future deterioration for each fuel cell stack; and a future output power estimator that estimates, for each fuel cell stack, a future output power value, which is a value of power that is likely to be outputted after a specific period of time has passed, based on the degree of future deterioration estimated by the deterioration estimator. The fuel cell stack combining method includes determining combinations of the fuel cell stacks based on differences in the output power value between the fuel cell stacks and differences in the future output power value between the fuel cell stacks.