To provide a ventilator, in which a volume of inlet air can be regulated by automatically regulating the rate of opening in accordance with the difference of pressures between a room and the outside.

A ventilation fan 7 is attached to a through hole 4 made in a partition wall 3 between a room 1 and open air 2. A mechanism 21 for regulating the rate of opening is provided in the inside of a tubular ventilator main body 11 fixed in the hole 4. The mechanism 21 increases automatically the rate of opening when the pressure difference between the room 1 and the open air 2 is increased.