Abstract: An inner peripheral portion of a receiving frame and an outer peripheral portion of a lid body each include first and second surface parts. In the closed-lid state, the first surface part of the lid body is supported by the first surface part of the frame, each of which include a gentle gradient. The second surface parts of the lid body and the receiving frame are pressed against each other by a force caused by elastic deformation of at least one of the outer peripheral portion of the lid body and the inner peripheral portion of the receiving frame. The second surface part of the receiving frame includes a steeper gradient than the first surface part of the receiving frame by reducing its diameter toward a downward direction of the receiving frame.

Abstract: An inner peripheral portion of a receiving frame includes: a receiving-frame first surface part with a gentle gradient, and a receiving-frame second surface part which is formed below the receiving-frame first surface part and with a steeper gradient than that of the receiving-frame first surface part by reducing its diameter toward a downward direction of the receiving frame, and an outer peripheral portion of a lid body includes: a lid first surface part with a gentle gradient; and a lid second surface part which is formed below the lid first surface part and either vertically toward a downward direction of the lid body, or with a steeper gradient than that of the receiving-frame second surface part by reducing its diameter toward the downward direction of the lid body, in which in a closed-lid state of the lid body, the lid first surface part is supported by the receiving frame first surface part, and the lid second surface part and the receiving-frame second surface part are pressed against each other by