To prevent the formation of a blow hole by introducing molten metal to the bottom portion of a relief part when a preform material is casted inside a crank lower case in a crankshaft supporting device.

This crankshaft supporting device is provided with a bearing retainer part retaining a bearing for supporting the crankshaft, the bearing support part is formed of an aluminum alloy, and the preform material is casted inside the aluminum alloy. The outer surface of the bearing retainer part is formed with a screw hole with an opened end part, the preform material is formed with the recessed relief part for storing the bottom part of the screw hole, and the relief part is provided with an introduction means introducing the molten metal into the inside in the casting.