An annular core piece in which an annular core piece can be received inside a contour of an outer circumstance of the annular core piece and the number of recesses formed on the outer circumstance is small is provided. In an annular core piece (1) including a plurality of recesses (5) opened in an outer circumferential part, the recess (5) includes a narrowed part (11) and a raised part (8), and a transverse width (W) of the narrowed part (11) is made narrower than a transverse width of a different region present on a side of a bottom of the recess (5) further beyond the narrowed part (11), and the raised part (8) is formed such that a part of the bottom of the recess (5) is raised and projected toward an opening end (6) of the recess (5).