To provide a window fixing device simple to operate by fitting a window half-opening device fitted with a support rod having a sucker at one end to a U-shaped metal fitting coated with a rubber liquid on the inside at a proper position of a window sash for natural ventilation.

A metal fitting 1 with a U-shaped cross section is fitted at an optional position of a plastic sash, and a sucker 3 is sucked to an opened window to keep it at the half-opened state. The metal fitting 1 is moved in the horizontal direction to adjust the opening angle of the window. The window is fixed in the opened state so that natural ventilation is ensured and the window is not opened or closed by a strong wind. A window fixing device simple to fit or remove and reliable to fix can be obtained.