To provide a power transmission component having well-balanced damping performance on natural vibration and other vibration propagated, and its manufacturing method for manufacturing the power transmission component in a relatively simple manner without complicating manufacturing processes and imposing excessive restrictions on manufacturing conditions.

An annular gear part structural member 3 is mounted in a casting mold while securing a casting cavity inside, and metal melt is filled into the cavity to integrally form an annular trunk 5 inside the gear part structural member 3. Thus, in a boundary area between the gear part structural member 3 an the trunk 5, a gear part (the power transmission component) is produced in which a metallurgic joint portion mutually connecting these with metallurgic joint and a contact interface portion having no metallurgic joint exist.