In a cross sectional plane perpendicular to a center axis line of an annular core, a connection projection and a connection recess have complementary configurations in which the connection projection and the connection recess are narrowed in width with distance from coupling surfaces, the connection projection and the connection recess respectively include a pair of friction surfaces and a pair of friction surfaces extending in a direction separate from the coupling surfaces, the friction surface of the connection projection is inclined relative to a virtual normal line perpendicular to a line connecting bottom portions of the pair of friction surfaces of the connection projection, and the friction surface of the connection recess is inclined relative to a virtual normal line perpendicular to a line connecting bottom portions of the pair of friction surfaces of the connection recess.