To provide a ventilation device that can be mounted to a new construction or an existing building without being restricted by the presence or absence of a wire screen.

A ventilation device is used for a window member that includes a frame body 92 that is provided along the opening of the building, and an opening/closing body 91 that is openably/closably disposed in the frame body 92 and has a ventilation rail 94 that communicates between the inside and the outside of a room to achieve ventilation. The ventilation device has a body part 2 that is disposed in the building or the frame body 92 having a first opening 23 opened to the inside of the building and a second opening 42 that is close at least to part of the ventilation rail 94 and faces it, an air blower 3 that is disposed in the body part 2 and can move air between the first opening 23 and the second opening 42, and a duct 4 that is provided in the body part 2 and located between the second opening 42 and the air blower 3.