PURPOSE: To prevent a risk in a corner section of a projection provided to the outside of the building by projecting a bay window to the outside of the building from a wall body, and protecting corner sections of the bay window with guards of a buffer material.

CONSTITUTION: Corner sections 6a and 7a of a bay window 1B are combined with guards 30 of a foam to fix, and in the case a person walks on, it can be prevented that the person runs against the corner sections 6a and 7a to get hurt. At the same time, it is prevented that fine splits of right and left mullions 6 and 7 of a bay window frame 2 catch clothes to have damage in the clothes. The guards 30 are fitted to recesses of the corner sections 6a and 7a of the bay window 1B so as to interlock with them, so that the slippage of the guards from the bay window 1B is easily prevented.