for only required parts although it may be made for the overall clutch housing for example referring to fig7 the respective inner surfaces of the cylindrical part approximately u00bd and the outer cylindrical part 11 the inner surface and the outer surface of the hub portion 2 are finished that is cutting is made only for a part which makes contact with a seal member which is not shown and a snap groove fig8 is an axially sectional view illustrating a clutch housing according to the present invention in a condition before finishing by cutting in order to show a thickness relationship among several parts of the clutch housing in fig8 t 1 denotes a thickness of the workipiece 1 which is a blank plate material t 2 a maximum thickness of a seal ring insertion portion t 3 a maximum thickness of a slide portion of the piston t 4 a thickness of the hub portion 2 and t 5 a thickness of the spline portion in order to satisfy predetermined requirements the wall thicknesses as stated above are set for example as follows although no specific limitation is made to the hardness of the workpiece 1 as a plate material it can expected that a workpiece 1 having a harness of not less than hv110 exhibits remarkable technical effects in addition to the above mentioned relationship further in view of the thickness relationship shown in fig8.