A base includes first to third projected portions. A sealing surface of the base is opposed to the housing and equipped with a sealing member. An attachment portion outside the sealing surface is screwed with the housing. A first projected portion projects from the attachment portion toward the housing. The second and third projected portions project relative to the sealing surface toward the housing. A recessed portion between the sealing surface and the first projected portion partially receives and fitted with a spigot protrusion of the housing. The second projected portion is located inside the third projected portion, and the third projected portion is located inside the recessed portion in the radial direction. The second and third projected portions extend along the circumferential direction. The first to third projected portions have projected tip end surfaces located in a same plane and in contact with the housing.