The room hygrostat serves for the supervision and control of the relative humidity in business premises, domiciles, habitations, conservatories, bathing rooms, swimming pools, EDP rooms, etc. The relative humidity prevailing in a room impacts on a special sensing strip that, upon the attaining of a certain value, triggers a changeover contact. The adjusting knob on the front of the device enables to adjust the desired set value. The setting range can be restricted.