A capacity detection type sensor element includes a rectangular vibrating plate, a flat back electrode which are provided opposedly with each other, and fixing portions which are provided adjoining to the vibrating plate, and has a predetermined length A of edge at a side adjoining to the vibrating plate. The back electrode is held by the fixing portions in a state that space is provided between the back electrode and the vibrating plate. Outer edges of the back electrode which are not held by the adjoining fixing portions, are straight lines and the straight lines define an octagonal shape as a whole.