An apparatus for window used for moving a movable window plate among an insertion position for being inserted into an aperture portion of a fixed window plate, an extraction position for being extracted from the aperture portion toward an indoor side, and an opening position for being moved from the extraction position in a sliding direction that is different from an inserting and extracting direction to open the aperture portion, the apparatus comprising: a rotating shaft rotatably supported at a predetermined position for the movable window plate; a movement conversion mechanism configured to convert a rotation of the rotating shaft into a translation of the movable window plate between the insertion position and the extraction position; and a drive shaft, fixed to the rotating shaft, configured to rotate the rotating shaft around a central axis of the rotating shaft by revolving around the central axis of the rotating shaft, wherein the movement conversion mechanism includes a cam fixed to the rotating shaft, and a fixture guide fixed to the fixed window plate, the fixture guide includes a cam guide groove for guiding the cam, and is in contact with an outer periphery of the cam on a wall surface of the cam guide groove on the indoor side, the wall surface of the cam guide groove on the indoor side is parallel to a main surface of the fixed window plate, and a distance between the wall surface of the cam guide grove on the indoor side and a central axis of the rotating shaft varies in response to a rotational angle of the rotating shaft, and wherein the cam is provided with an extension shaft of the rotating shaft, and the fixture guide has a shaft guide groove for guiding the extension shaft on a groove bottom surface of the cam guide groove, characterized in that the apparatus for window further includes an elastic member configured to press the extension shaft against a wall surface of the shaft guide groove on an outdoor side, and press the cam against the wall surface of the cam guide groove on the indoor side, while the movable window plate is slid between the extraction position and the opening position. a rotating shaft rotatably supported at a predetermined position for the movable window plate; a movement conversion mechanism configured to convert a rotation of the rotating shaft into a translation of the movable window plate between the insertion position and the extraction position; and a drive shaft, fixed to the rotating shaft, configured to rotate the rotating shaft around a central axis of the rotating shaft by revolving around the central axis of the rotating shaft, wherein the movement conversion mechanism includes a cam fixed to the rotating shaft, and a fixture guide fixed to the fixed window plate, the fixture guide includes a cam guide groove for guiding the cam, and is in contact with an outer periphery of the cam on a wall surface of the cam guide groove on the indoor side, the wall surface of the cam guide groove on the indoor side is parallel to a main surface of the fixed window plate, and a distance between the wall surface of the cam guide grove on the indoor side and a central axis of the rotating shaft varies in response to a rotational angle of the rotating shaft, and wherein the cam is provided with an extension shaft of the rotating shaft, and the fixture guide has a shaft guide groove for guiding the extension shaft on a groove bottom surface of the cam guide groove,