To provide a window open-close device, in which a labor-hour required for manufacture is lowered and cost of which is reduced.

A casing, in which one surface is opened, is fixed onto a window frame. An operating shaft 18, which is penetrated to the closing wall of the easing so that one end section is placed in the casing while being freely rotated around an axis, is installed to the easing. A worm 21a is fastened at one end section of the operating shaft 18, and a worm wheel 13 is mounted into the casing so as to be engaged with the worm 21a. A rotating arm 14, which is turned integrally with the worm wheel 13 and in which at least a part is projected from the opening 11a of the casing, is set up to the worm wheel 13. A sash is opened and closed by the rotation of the rotary arm 14 with the revolution of the operating shaft 18. The casing is composed of a box-shaped main body 10, in which one surface is opened and which is made of plastics, and a reinforcing member 11 fixed into the main body 10 and made of a metal, and the worm wheel 13 is supported by the reinforcing member 11.