PURPOSE: To restrain noise and obtain a mild warm air flow of a low flow rate by a method wherein when fan operation mode is set at 'silent' mode, operation mode is changed over either to stop of fan operation or parallel operation of two or more fans at low rotating speed.

CONSTITUTION: When a silent mode setting device 6c is turned on, fan operation mode is changed over either to '2' ('WEAK'), '1' ('VERY WEAK') or '0' ('OFF'). Then fan operation mode is kept at '2' ('WEAK') until the room temperature rises and reaches Ts+1.2°C. When the room temperature reaches Ts+1.2°C, the fan operation mode is changed over from '2' ('WEAK') to '1' ('VERY WEAK'), and when the room temperature reaches Ts+2.8°C, it is changed over from '1' ('VERY WEAK') to '0' ('OFF'). When the room temperature is falling, that is, when the fan operation mode is '1' ('VERY WEAK') or '0' ('OFF'), and the room temperature further drops to Ts+0.4°C, the fan operation mode is changed over to '2' ('WEAK'). Thus, while the silent mode setting device 6c is turned on, the fan operation mode is prevented to be changed over to '4' ('STRONG') or '3' ('MEDIUM') but changed over only to '2' ('WEAK') '1' ('VERY WEAK') or '0' ('OFF'), so that generation of rasping noise can be restrained.