A window glass breakage detection tool attached to a window glass of a vehicle to detect breakage of the window glass is disclosed. When the window glass is unbroken, at least part of the breakage detection tool is located in a through hole arranged in the window glass and urges a wall of the through hole in a direction expanding the through hole with an elastic force of the detection tool. When the window glass breaks, the breakage detection tool shatters the window glass at a portion surrounding the through hole and the detector is at least partially displaced.