To easily perform work for installing a slip-out preventive piece 6 in an upper frame 1 for preventing the slip-out of a sliding door sash window by preventing the floating of the sliding door sash window.

This slip-out preventive piece 6 is installed in a locking groove 10 arranged on the under surface of the upper frame 1 in a factory in a state of being indicated in (A). At a construction site, the slipping-out preventive piece 6 is rotated in a state of (B) after building up the sliding door sash window in a window frame. In this state, an abutting part 15 of the slip-out preventive piece 6 oppositely approaches the upper edge of the sliding door sash window.