This study investigates subjective evaluation of transmitted speech sound from open windows. When windows are opened, sound pressure level difference decreases, but background noise increases in sound receiving room. This paper describes the results of an experiment under conditions of open windows. The results of this paper are as follows. 1. When windows are opened, background noise increases in sound receiving room, yet point of subjective evaluation for sound insulation decreases. 2. The relation of word intelligibility to level difference between transmitted speech sound and background noise level shows a defferent tendency in comparison with conditions of closed windows.