A sensor case includes a body portion having a circular horizontal section. An upper body 1 includes an accommodation space 4 having a wall surface having a circular horizontal section, and the body portion is inserted in the accommodation space 4 such that the body portion is capable of rotating about a central axis thereof extending in a vertical direction. A stopper 50 is arranged to project in a horizontal direction in an outer circumferential surface of the body portion. A guide groove 51 is arranged to extend in the vertical direction at the wall surface of the accommodation space 4, and is arranged to allow the stopper 50 to move therein. The upper body 1 includes a restricting portion 52 arranged to restrict a vertical movement of the stopper 50.