A reactor for moisture generation completely preventing hydrogen gas from being ignited, back fire to a gas feed source side from occurring, and a platinum coating catalyst layer from being peeled off inside a reactor main body for moisture generation so as to further increase the safety of the reactor for moisture generation and reduce a dead space of an internal space inside the reactor main body in order to further reduce the size of the reactor main body, wherein an inlet side reactor main body member (1) having a gas feed port (1a) and an outlet side reactor main body member (2) having a moisture gas outlet port (2a) are combined opposedly to each other, these both members are welded with each other so as to form a reactor main body A, a reflector is provided inside the internal space (V) of the reactor main body, a platinum coating catalyst layer (8) is formed on the inner wall surface of the outlet side reactor main body member (2), and hydrogen and oxygen fed from the gas feed port (1a) to the internal space (V) of the reactor main body A are brought into contact with the platinum coating film (8b) to activate its reactivity, whereby hydrogen and oxygen are reacted under noncombustion state so as to produce water.