PURPOSE: To operate a bathroom ventilator with a required minimum power consumption and prevent growth of mold by a method wherein a piece of white cement, the same material as masonry joint of tile walls, is attached at a position ay far away enough from the ventilator and the electric resistance value of the white cement is measured.

CONSTITUTION: A humidity sensor 3 has such a construction that a piece of white cement 1 attached with two electrodes 2 is molded in a resin case 5 with resin 6. The piece of white cement 1 is 3mm in width, 4mm in thickness and 20mm in length, and the two electrodes 2 are embedded in the middle of the piece of white cement 1 at a distance of 40mm. The humidity sensor 3 is mounted on a ceiling or a wall of a bathroom, and is controlled by a controller provided in a casing of a bathroom ventilator or an automatic bath controller. The controller is equipped with a power source circuit which applies direct current voltages across the electrodes 2 of the humidity sensor 3, a voltage comparison circuit which detects such a fact that the electric resistance across the electrodes 2 reaches a specified value, and relays through which a ventilator is started and stopped according to outputs of the voltage comparison circuit.