PURPOSE: To improve the thermal fatigue resistance of plug hole parts by forming a trochoid-shaped metal cylindrical boy having a number of projections on the outer surface, Al alloy basic member surrounding the outer peripheral part of the cylindrical body and plug holes which penetrate through the both bodies.

CONSTITUTION: In manufacture, a number of projections 3 are formed over the whole area on the outer surface of a trochoid-shaped cylindrical body 2. Then, in order to form the bottomed casting holes 13 and 14 in correspondence with the plug hole formation part of a rotor housing 1, a core is arranged at the corresponding part. Then, the molten material of Al alloy is casted into the outer peripheral part of the cylindrical body 2, and after coagulation, a casting mold and the core are demounted, and an Al alloy basic member 4 is joined onto the outer peripheral surface of the cylindrical body 2 by utilizing the projections 3, and the bottomed casting holes 13 and 14 are formed at prescribed positions. Then, the Al alloy basic member 4 on the peripheral edge of the plug hole corresponding to the bottom parts of the casting holes 13 and 14 is locally fused and then chill to form minutely fine particles, and a chilled structural part 8 is formed, and then plug holes 6 and 7 are drilled at the prescribed positions.