An upper end of a link is attached to a roller shaft of an upper guide roller provided in an upper part of a front window for opening and closing a front opening of a cabin. A lower end of the link is provided with a wire retaining pin that moves in a wire guide rail. One end of an assist wire connected to a spring at the other end is connected to the wire retaining pin. Thus, in a window-closed state, tension acts on the front window as a pressing force for pressing the front window forward, and when the front window is to be opened, the tension acts through the link on the front window as an assisting force.
