To provide an air vent device capable of discharging only surplus air without discharging water or hot water at all, and being easily and cheaply manufactured.

An air vent device consists of a casing 21 in which an ebb and flood chamber 21A provided with a hot water entrance 22 for receiving hot water is provided at the lower part, a ventilation chamber 21B provided with an atmosphere port 24 for drawing surplus air is provided at the right place above the ebb and flood chamber 21A, and a connecting hole 21h connecting the ebb and flood chamber 21A to the ventilation chamber 21B is provided, a spherical float 35 put into the ebb and flood chamber 21A floating in the water, and a moving body 30 provided with an opening and closing valve 32V provided above the spherical float 35 so that it can freely rise and fall, and is usually lowered by an energizing means, for example, a spring 30S and elevated by the buoyancy of the spherical float 35 and which can freely open and close the connecting hole 21h. When the moving body 30 is lowered, the connecting hole 21h is open, and when the moving body 30 is elevated, the connecting hole 21h is closed by the operating and closing valve 32V.