A bearing device comprising: a rolling bearing having an inner ring, an outer ring having a small-diameter step portion provided on an outer periphery of an axial end portion thereof, and a plurality of rolling elements arranged to be rollable between the inner ring and the outer ring, and a retainer plate having a fitting hole to be fitted to the small-diameter step portion and configured to fix the rolling bearing to a housing, wherein an outer peripheral surface of the small-diameter step portion is formed with an engagement groove extending in a circumferential direction, wherein the retainer plate has a plurality of engagement claws protruding radially inward from an inner periphery of the retainer plate defining the fitting hole and configured to be engaged to the engagement groove, and wherein in a state that the fitting hole is fitted to the small-diameter step portion, the engagement claws are formed by axially pressing the inner periphery of the retainer plate, including at least thickness reduction parts formed in advance at a peripheral edge between an inner peripheral surface of the fitting hole and a side surface of the retainer plate, and plastically deforming the inner periphery of the retainer plate so as to protrude radially inward, and the engagement claws are engaged to the engagement groove, characterized in that the fitting hole has a plurality of relief parts of which an inner diameter is greater than other part, and that each of the thickness reduction parts has an arc length shorter than an arc length of the relief part and is formed at a peripheral edge between an inner peripheral surface of the relief part and the side surface of the retainer plate. a rolling bearing having an inner ring, an outer ring having a small-diameter step portion provided on an outer periphery of an axial end portion thereof, and a plurality of rolling elements arranged to be rollable between the inner ring and the outer ring, and a retainer plate having a fitting hole to be fitted to the small-diameter step portion and configured to fix the rolling bearing to a housing, wherein an outer peripheral surface of the small-diameter step portion is formed with an engagement groove extending in a circumferential direction, wherein the retainer plate has a plurality of engagement claws protruding radially inward from an inner periphery of the retainer plate defining the fitting hole and configured to be engaged to the engagement groove, and wherein in a state that the fitting hole is fitted to the small-diameter step portion, the engagement claws are formed by axially pressing the inner periphery of the retainer plate, including at least thickness reduction parts formed in advance at a peripheral edge between an inner peripheral surface of the fitting hole and a side surface of the retainer plate, and plastically deforming the inner periphery of the retainer plate so as to protrude radially inward, and the engagement claws are engaged to the engagement groove, the fitting hole has a plurality of relief parts of which an inner diameter is greater than other part, and that each of the thickness reduction parts has an arc length shorter than an arc length of the relief part and is formed at a peripheral edge between an inner peripheral surface of the relief part and the side surface of the retainer plate.