To provide an inhaler capable of automatically controlling the flow of a fresh gas in a short time when a ventilation mode is switched.

This inhaler is provided with a loop-like gas circuit 3 having an aspiratory branch 1 and a respiratory branch 2; a manual ventilation balloon 10; mechanical ventilation bellows 14; a switching means 20 controlling the gas path between the ventilation balloon 10 or the ventilation bellows 14 and the gas circuit to switch a mechanical ventilation mode and a manual ventilation mode; and a driving means controlling a feed control means of the fresh gas to introduce a prescribed quantity of the fresh gas after the switching means 20 is operated according to the determination of the finish point of an aspiratory phase.