To prevent dew condensation by arranging a fan at the lower end side between a curtain and window pane and sending air into a space between the curtain and the window pane by the fan to generate air convection.

A curtain 2 is disposed in the indoor side R of a window pane 1. A fan 3 is constituted of a fan body 30 provided with a built-in fan driven by a motor and a delivery duct 31 extended along from the fan body 30 to the lower edge 1a of the window frame and closed at the extended end 31a and provided with an exhaust air aperture 32 at an appropriate position. The fan 3 is disposed at the lower end side 1a between the curtain 2 and the window pane 1 to send air into the space S between the curtain 2 and the window pane 1 to generate air convection. Even when a curtain is disposed on the window face, a breeze is sent into the space between the curtain and the window face to generate air convection. Hence, dew condensation on a window frame or the like covered with a curtain can be prevented.