To realize a moisture sensing of a high reliability at a low cost even in a box body under a steamed state.

A wet bulb temperature sensor 31 is always kept wet by sucking up water in a water tank 26 by a hygroscopic material 33. By a sensed temperature difference between the wet bulb temperature sensor 31 which senses a wet temperature in the box body 1, and a dry bulb temperature sensor 32 which senses a dry temperature in the box body 1, a relative moisture in the box body 1 can be calculated. Thus, by utilizing the wet bulb temperature sensor 31 and the dry bulb temperature sensor 32 which are temperature detecting means, a simple and reliable moisture measurement is realized.