A slender-type diaphragm longer in a longitudinal direction and shorter in a transversal direction in relation to the longitudinal direction, has a base member and a slant member. The base member has a pair of extending members provided in parallel in the longitudinal direction as facing each other, and a curved section that protrudes toward a first side of the diaphragm in a vibration direction of the diaphragm from a first plane that includes the extending members, the vibration direction being perpendicular to the longitudinal and transversal directions. The slant member has a slant surface in relation to the first plane and connected to the base member in a manner that the slant member surrounds the base member in a second plane parallel to the first plane, the slant member having a curved surface in a cross section thereof in relation to a cross section of the base member in the transversal direction.