A bearing device comprising: a rolling bearing including a plurality of rolling elements arranged between an inner ring and an outer ring, and a retainer plate configured to retain the rolling bearing to a housing and having a circular hole portion to be fitted to an outer peripheral surface of the outer ring of the rolling bearing, wherein an outer peripheral portion of an axial end portion of the outer ring is provided with a small-diameter step portion to be fitted with the retainer plate, and an outer peripheral surface of the small-diameter step portion is formed with an engagement groove extending in a circumferential direction, wherein an inner peripheral portion of the circular hole portion of the retainer plate is provided with at least three cutout portions, and each of the cutout portions is formed with an engagement protrusion protruding radially inward to be engaged to the engagement groove, wherein an inner diameter-side tip portion of each of the engagement protrusions has a concave shape, wherein at least both circumferential ends of the inner diameter-side tip portion overlap with the engagement groove even at a state where the outer ring and the retainer plate are eccentric or inclined, as seen from an axial direction, wherein the inner diameter-side tip portion of each of the engagement protrusions has a circular arc shape, wherein a radius of curvature of the inner diameter-side tip portion is equal to a radius of curvature of an outer peripheral surface of the small-diameter step portion of the outer ring and substantially the same as the radius of curvature of the cutout portion. a rolling bearing including a plurality of rolling elements arranged between an inner ring and an outer ring, and a retainer plate configured to retain the rolling bearing to a housing and having a circular hole portion to be fitted to an outer peripheral surface of the outer ring of the rolling bearing, wherein an outer peripheral portion of an axial end portion of the outer ring is provided with a small-diameter step portion to be fitted with the retainer plate, and an outer peripheral surface of the small-diameter step portion is formed with an engagement groove extending in a circumferential direction, wherein an inner peripheral portion of the circular hole portion of the retainer plate is provided with at least three cutout portions, and each of the cutout portions is formed with an engagement protrusion protruding radially inward to be engaged to the engagement groove, wherein an inner diameter-side tip portion of each of the engagement protrusions has a concave shape, wherein at least both circumferential ends of the inner diameter-side tip portion overlap with the engagement groove even at a state where the outer ring and the retainer plate are eccentric or inclined, as seen from an axial direction, wherein the inner diameter-side tip portion of each of the engagement protrusions has a circular arc shape, wherein a radius of curvature of the inner diameter-side tip portion is equal to a radius of curvature of an outer peripheral surface of the small-diameter step portion of the outer ring and substantially the same as the radius of curvature of the cutout portion.