A connecting structure which connects a first member and a second member, wherein: said first member comprises a connecting upright portion which is projected in a substantially cylindrical shape; and said second member comprises a connecting side-wall portion which overlies said connecting upright portion and a plurality of locking projections provided to extend inwards from the inner side surface of the connecting side-wall portion; said first member comprises a plurality of locking flanges (25, 26) provided to extend outwards along the circumferential direction of the outer side surface of the connecting upright portion; and a bottom face of each of said locking flanges (25, 26) distends downwards in a convex curved shape and forms a convex curved bottom face along a circumferential direction of said connecting upright portion, and when said second member overlies said first member to be rotated, upper faces of said locking projections slide and contact with said convex curved bottom faces of said locking flanges (25, 26) to be guided on said convex curved bottom faces and locked at the lowermost position of the convex curved bottom faces; characterized in that; said first member is not provided with a part which contacts with a lower end of the locking projections; and wherein said first member comprises two or more groups of locking flanges (25, 26) whose locking forces vary, composed of a plurality of locking flanges (25, 26) provided in positional relationship to be reciprocally engageable with said locking projections, and each of the groups of locking flanges (25, 26) is formed so that a dimension from an upper end surface of said connecting upright portion to the lowermost position of said convex curved bottom faces varies according to groups of locking flanges (25, 26). said first member comprises a connecting upright portion which is projected in a substantially cylindrical shape; and said second member comprises a connecting side-wall portion which overlies said connecting upright portion and a plurality of locking projections provided to extend inwards from the inner side surface of the connecting side-wall portion; said first member comprises a plurality of locking flanges (25, 26) provided to extend outwards along the circumferential direction of the outer side surface of the connecting upright portion; and a bottom face of each of said locking flanges (25, 26) distends downwards in a convex curved shape and forms a convex curved bottom face along a circumferential direction of said connecting upright portion, and when said second member overlies said first member to be rotated, upper faces of said locking projections slide and contact with said convex curved bottom faces of said locking flanges (25, 26) to be guided on said convex curved bottom faces and locked at the lowermost position of the convex curved bottom faces; characterized in that; said first member is not provided with a part which contacts with a lower end of the locking projections; and wherein said first member comprises two or more groups of locking flanges (25, 26) whose locking forces vary, composed of a plurality of locking flanges (25, 26) provided in positional relationship to be reciprocally engageable with said locking projections, and each of the groups of locking flanges (25, 26) is formed so that a dimension from an upper end surface of said connecting upright portion to the lowermost position of said convex curved bottom faces varies according to groups of locking flanges (25, 26).