A bearing device comprising: a rolling bearing having an inner ring, an outer ring and a plurality of rolling elements arranged between the inner ring and the outer ring, and a retainer plate having a substantially circular hole, a plurality of attachment holes provided around the substantially circular hole and configured to enable each of fastening members to be inserted or screwed thereinto, and a plurality of boss parts formed with the attachment holes, wherein the retainer plate is configured to be in contact with the outer ring in an axial direction, wherein the rolling bearing is fixed to a housing via the retainer plate by fitting the outer ring to the housing and tightening each fastening member to fix the retainer plate to the housing, wherein a contact surface of the outer ring with which the retainer plate is in contact in the axial direction is positioned at a more axially outboard side by a predetermined distance than a facing surface of the housing that the retainer plate faces, wherein the retainer plate is formed with rigidity reducing parts for reducing rigidity of the retainer plate at radially outermore sides than an outer peripheral surface of the outer ring between the substantially circular hole and the attachment holes, and wherein the rigidity reducing parts are through-holes, each of which is line-symmetrically formed with respect to a virtual line connecting a center of the substantially circular hole and a center of each attachment hole, or concave portions (45A, 45B), each of which is line-symmetric with respect to the virtual line and is formed from at least one of a front side and a back side of the retainer plate, characterized in that each of the through-holes or concave portions (45A, 45B) has a substantial M shape that is defined by a circular arc formed along an inner diameter of the fitting hole, a circular arc formed along an outer diameter of the boss part and two sides connecting end portions of the circular arcs (E, G). a rolling bearing having an inner ring, an outer ring and a plurality of rolling elements arranged between the inner ring and the outer ring, and a retainer plate having a substantially circular hole, a plurality of attachment holes provided around the substantially circular hole and configured to enable each of fastening members to be inserted or screwed thereinto, and a plurality of boss parts formed with the attachment holes, wherein the retainer plate is configured to be in contact with the outer ring in an axial direction, wherein the rolling bearing is fixed to a housing via the retainer plate by fitting the outer ring to the housing and tightening each fastening member to fix the retainer plate to the housing, wherein a contact surface of the outer ring with which the retainer plate is in contact in the axial direction is positioned at a more axially outboard side by a predetermined distance than a facing surface of the housing that the retainer plate faces, wherein the retainer plate is formed with rigidity reducing parts for reducing rigidity of the retainer plate at radially outermore sides than an outer peripheral surface of the outer ring between the substantially circular hole and the attachment holes, and wherein the rigidity reducing parts are through-holes, each of which is line-symmetrically formed with respect to a virtual line connecting a center of the substantially circular hole and a center of each attachment hole, or concave portions (45A, 45B), each of which is line-symmetric with respect to the virtual line and is formed from at least one of a front side and a back side of the retainer plate, characterized in that each of the through-holes or concave portions (45A, 45B) has a substantial M shape that is defined by a circular arc formed along an inner diameter of the fitting hole, a circular arc formed along an outer diameter of the boss part and two sides connecting end portions of the circular arcs (E, G).