NOVELTY - Controlling reaction conditions of fuel in a fuel cell (1) of fuel cell system comprises obtaining information regarding current external environment of fuel cell system; determining target value of each of factors which determine reaction conditions of the fuel in the fuel cell; and controlling peripheral devices of fuel cell which affect reaction conditions of fuel in the fuel cell according to target value of each of the factors. Determination of target value is based on correspondence relationship of change of each factor with respect to change in external environment of fuel cell system.    USE - Method for controlling reaction conditions of fuel in a fuel cell of fuel cell system.    ADVANTAGE - Temperature and fuel concentration controlling is performed rapidly.    DETAILED DESCRIPTION - Controlling reaction conditions of fuel in a fuel cell (1) of fuel cell system comprises obtaining information regarding current external environment of fuel cell system; determining target value of each of factors which determine reaction conditions of the fuel in the fuel cell; and controlling peripheral devices of fuel cell which affect reaction conditions of fuel in the fuel cell according to target value of each of the factors. Target value is varied corresponding to obtained information. Determination of target value is based on correspondence relationship of change of each factor with respect to change in external environment of fuel cell system. INDEPENDENT CLAIMS are included for:    (1) fuel cell system which comprises fuel cell to produce power by using fuel; Balance of Plant (BOP) (2) to supply at least fuel to the fuel cell; and controller (3) to simultaneously control peripheral devices of fuel cell in the BOP according to varying target value of each of factors determining reaction conditions of fuel in the fuel cell in a current external environment of the fuel cell system; and    (2) a computer-readable recording medium which comprises program for executing, in a computer the above method.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic diagram illustrating block diagram of fuel cell system.    Fuel cell (1)    Balance of plant (2)    Controller (3)    Battery (6)    Load (7)