To provide a separate type window screen which can prevent creation of a gap between the window screen and a slide door in order to completely prevent an insect from entering, which can ensure ventilation in such a condition that doors are partly opened on opposite sides of a window in a window frame in which two slide doors are incorporated, and which can ensure ventilation through three parts, that is, opposite side parts and the center part of a window frame in which four slide doors are incorporated.

A window screen incorporates sash rollers which are fitted to a door frame including an upper and lower window screen rails provided on the outside of a slide door T. Two of such window screens in a set have a width W which is about one-half of the width L of the slide door.