To provide a method for controlling an expiratory valve provided at a ventilator between exhalations.

During a first interval t1, one or both of the parameters of a flow rate passing through an expiratory valve 20 and pressure at the valve 20 are measured to decide when these parameters satisfied a prescribed condition. When the parameters satisfy the condition, a second interval t2 is started and thee valve 20 is adjusted during a second interval t2 to attain a previously set terminal pressure PEEP in an exhalation division.