PURPOSE: To generate the air with stable target humidity by sucking surrounding air by using compressed dry air as a power source and mixing the compressed dry air with the surrounding air.

CONSTITUTION: The high pressure dry air is jetted from a nozzle 1 through a high pressure dry air supply line 6, a pressure control valve 5 for high pressure dry air, and a flow control valve 4 for high pressure dry air to reduce the pressure of air near the inner wall of a diffuser 2 so that room air is sucked through a room air suction port 9, a room air suction flow control valve 8, and a suction room air flow meter 7, and the high pressure dry air and the room air are mixed in the diffuser 2. The humidity and the flow rate of this mixed air are adjusted respectively by valves 4, 5, 8 while checking the flow rate by a mixed gas flow meter 10 to give a target value. Thus, the air adjusted with humidity and flow rate is blown out of a mixed gas outlet port 11.