To provide a magnetic encoder in which a base material has a ring-shaped body portion into which a rotation axis is inserted when a magnetic drum is mounted on a rotation axis, and an engagement convex portion provided so as to project over a whole circumference of an outer periphery of the body portion. The body portion has a centering track provided in a part of the outer periphery of the body portion over the whole circumference as a reference of circular runout tolerance on an outer periphery of a magnet. The engagement convex portion has a constricted portion projecting from the outer periphery of the body portion, and a cylindrical portion extending in an axial direction of the body portion from a tip end of the constricted portion. The magnet surrounds the engagement convex portion so as to wrap around to a gap between the cylindrical portion and the body portion, and exposes the centering track without covering the centering track.