NOVELTY - A reaction system comprises catalytic reaction device (4) which has two entrance and exits, reaction flow path provided in other side from one side among two entrance and exits and catalyst provided in reaction flow path, upstream processor (3) which processes fuel and sends out processed material to catalytic reaction device from exit, and switch part which selectively switches first state that connected other side to entrance of downstream reactor (5) to second state that connected one side to entrance of downstream reactor.    USE - A reaction system for producing hydrogen, and fuel cell power generating devices e.g. portable electronic devices e.g. notebook personal computer, personal digital assistant (PDA), electronic notebook, digital camera, mobile telephone, wristwatch and game machine device.    ADVANTAGE - The reaction system continuously performs catalytic reaction by catalytic reaction device.    DETAILED DESCRIPTION - A reaction system comprises catalytic reaction device (4) which has two entrance and exits, reaction flow path provided in other side from one side among two entrance and exits and catalyst provided in reaction flow path, upstream processor (3) which processes fuel and sends out processed material to catalytic reaction device from exit, downstream reactor (5) which takes in product that passed through catalytic reaction device from entrance and reacts taken in product, and switch part which selectively switches first state that connected other side to entrance of downstream reactor while connecting one side to exit of upstream processor among two entrance and exits to second state that connected one side to entrance of downstream reactor while connecting other to exit of upstream processor.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the fuel cell power generating devices. (Drawing includes non-English language text).    Fuel container (1)    Fuel feed pump (2)    Upstream processor (3)    Catalytic reaction device (4)    Downstream reactor (5)