To increase the active effect of reaction agents in a hydrogen generator.

A reaction vessel 1 made of stainless steel comprises: a disk-shaped outside case 2 with a large diameter; an inside case 3 with an intermediate diameter stored therein; and a heater pipe 4 with a small diameter disposed at the central part of the inside case 3. An electric heater 5 as an inside heating apparatus is inserted into the heater pipe 4, the circumferential face of the outside case 2 is heated by an outside heating apparatus 6, reaction agent receptors 7 and 8 are respectively arranged at the central part in the longitudinal direction of a first space S₁ between the outside case 2 and the inside case 3 and a second space S₂ between the inside case 3 and the heater pipe 4, the reaction agent receivers 7 and 8 are located at the central part of a reaction cell 1 and is made to correspond to heating parts, water tubes 9 and 10 feeding water to the first and second spaces S₁, S₂ act as jet tubes of heated water vapor upon the cleaning of teh reaction agents, the heating of the reaction agents is uniformly performed from the inside and the outside, and further, the cleaning of the reaction agents is performed to increase the activity of the reaction agents.