Abstract: A magneto rotor has a cup-shaped iron casing accommodating magnets disposed along and in contact with the inner peripheral surface of the casing. The magnets form a circumferentially discontinuous but generally circular annulus into which a generally annular magnet retainer member is forced through the open end of the casing to radially outwardly urge the magnets into face-to-face engagement with the casing inner peripheral surface, to retain the magnets in position therein. The magnet retainer member has an axially inner fixing section fixed to the casing bottom wall and ridges radially outwardly projecting from a cylindrical section of the retainer member into spaces between the magnets. When the magnet retainer member is forced into the annulus of magnets, the cylindrical section is radially outwardly deformed at the ridges.