<p>In the present invention, a substrate (11) has: a ring-shaped main body section (111) into which a rotating shaft is inserted when a magnetic drum is mounted to the rotating shaft; and an engagement protrusion (112) that protrudes across the entire outer peripheral surface of the main body section (111). The main body section (111) has a centering track (111a) provided across the entire periphery of a portion of the outer peripheral surface of the main body section (111) as the baseline for the radial run-out tolerance of the outer peripheral surface of a magnet. The engagement protrusion (112) has: a constricted section (112b) protruding from the outer peripheral surface of the main body section (111); and a tubular section (112a) extending in the axial direction of the main body section (111) from the tip of the constricted section (112b). The magnet (12) encircles the engagement protrusion (112) in a manner so as to wrap around the gap between the tubular section (112a) and the main body section (111), and does not cover the centering track (111a), causing the centering track (111a) to be exposed.</p>
