A gasket 36 is interposed between an end face 341 of a front housing 11 and an end face 351 of a rear housing 12. A coned disc spring 37 is interposed between an end face 192 of a cylinder 19 and the end face 341 of the front housing 11. When the end faces 341, 351 are caused to approach each other so as to be joined together, the coned disc spring 37 is first held by the end face 341 of the front housing 11 and the end face 192 of the cylinder 19. When the end faces 341, 351 are caused to approach each other further so as to be joined together, the gasket 36 is held between the end faces 341, 351. Thus, it is ensured that the cylinder and the seal material interposed between the first housing and the second housing are held therebetween.