PURPOSE: To detect leakage of reaction gas to a heating medium when the leakage occurs by a method wherein a detecting means to detect reaction gas leaking to a heating medium is situated in the heating medium in a chemical reactor.

CONSTITUTION: Gas introduced through a reaction gas inflow and outflow port 2 to a chemical reactor 1 flows through a reaction gas flow passage 3 to a reaction material 7, wherein the gas is reacted to generate heat. The heat is transmitted through a heat transfer surface 8 and a fin 9 to a heating medium, e.g. water, flowing through a heating medium flow passage 6, and removed. Meanwhile, during reverse reaction, a reaction material 7 is heated through the heat transfer surface 8 and the fin 9 by mans of a heating medium flowing through the heating medium flow passage 6 to generate reaction gas. The reaction gas is guided through the inflow and outflow port 2 to the outside. When, during the reverse reaction, the reaction gas leaks past a part of the reaction gas flow passage 3 and the heat transfer surface 8 to the heating medium, in the case of the reaction gas being water soluble gas, the reaction gas is dissolved in water being the heating medium and flows in the direction of an outlet 5. The dissolved reaction gas is detected by means of a pH sensor 10, and leakage is detected by a signal transmitter 11.