Stator core and method of manufacturing same

A method of manufacturing a stator core formed by laminating stator core sheets, each of the stator core sheets punched out from a magnetic metal sheet, a central portion of the magnetic metal sheet previously punched out to form a rotor core sheet, the stator core sheet having a ring-shaped yoke piece and magnetic pole pieces formed radially inside the yoke piece, each of the magnetic pole pieces having a straight piece and a magnetic pole teeth piece formed radially inside the straight piece, the method comprising the steps of

• pressing one portion of the straight piece from both sides in a thickness direction to form a thin section;
• forming grooves at both sides in a thickness direction of the thin section, the grooves having the same rectangular cross-sectional shape, depths of the grooves being within a range of 2-20% of a thickness of the magnetic pole piece;
• and
• radially-inwardly elongating the magnetic pole piece by a predetermined length.