Abstract: A casing body made by forming a synthetic resin into a bottomed cylinder-like configuration with a first bracket integrally provided at one end of a cylindrical portion. The cylinder body includes a plurality of clamping portions connected at their base ends to the first bracket at locations equally spaced circumferentially of the cylindrical portion, and magnet retaining portions each protruding to opposite sides from a tip end of each clamping portion. When the casing body is formed by molding, a yoke and a plurality of magnets are integrally coupled to each other. The yoke is clamped between the clamping portions and the cylindrical portion, and the magnets abut against an inner surface of the yoke with their circumferential opposite side ends being defined by the clamping portions and with their axially opposite ends being defined by the first bracket and the magnet retaining portions.