A cooling portion has an air inlet into which cooling air flows and is positioned in a preset range. When a distance from the upper to lower ends of an oil chamber in the axial direction of a rotating shaft member is h, and the shortest distance between an outer peripheral face of a casing and an inner peripheral face of a cover is a, the preset range is set in the following manner. Specifically, the preset range is set between ¼h upward and 5/4h downward relative to the upper end of the oil chamber as a reference point, in the axial direction of the rotating shaft member. Additionally, the preset range is set between the outer peripheral face of the casing and relative to the outer peripheral face of the casing as a reference point, in the radial direction of the rotating shaft member.