A core block has an arm portion, and a yoke portion extending circumferentially at an end of the arm portion. At least the yoke portion is configured such that a plurality of core segments are stacked in multiple levels in an axial or radial direction. Each of the core blocks has engaging portions formed at respective opposite ends of the yoke portion and adapted to connect adjacent core blocks. In a first-level core segment, each of the engaging portions has a protrusion extending circumferentially from a corresponding one of the opposite ends of the yoke portion, and a recess juxtaposed radially to the protrusion and adapted to receive the protrusion of an adjacent core block. In a second-level core segment, each of the engaging portions has the protrusion and the recess provided in positionally reversed relation with those of the first-level core segment with respect to the radial direction.