To reduce the cost by decreasing parts requiring accuracy and labor in upper and lower shells which are respectively provided with an outside wall and an inside wall on both sides in an axial direction and where a shaft assembly and a turning door are respectively rotatably held by bearing parts respectively formed on the inside walls of the upper and the lower shells.

The upper and the lower shells 6 and 7 are respectively provided with the outside walls 6a, 6b, 7a and 7b and the inside walls 6c, 6d, 7c and 7d on both sides in the axial direction; and the shaft assembly is rotatably held by the bearing parts formed on the inside walls 6c, 6d, 7c and 7d, and the turning door 8 is held to be opened/closed by the bearing parts formed on the inside walls 6c, 6d, 7c and 7d or an orthogonal wall successively provided to the inside of the inside wall. The divided surfaces of a barrel wall between the inside walls of the shells 6 and 7 have butting surfaces which are brought into contact with each other when the shells 6 and 7 are combined, and the divided surfaces on the outside in the axial direction from the inside walls of the shells 6 and 7 have opposed surfaces which are opposed to each other through a gap when the shells 6 and 7 are combined.