A rolling bearing unit with retainer plate comprising: an outer ring (5a, 5b) having an outer ring raceway around an inner-circumferential surface thereof; an inner ring arranged to be concentric with the outer ring (5a, 5b) and having an inner ring raceway around an outer-circumferential surface thereof; plural rolling bodies provided between the outer ring raceway and the inner ring raceway to rotate freely; and a retainer plate (7a-7c) having a support hole (13a-13c) and plural mounting holes (10, 10a, 10b) that are provided around the support hole (13a-13c), the outer ring (5a, 5b) further having: a small-diameter stepped section (9a, 9b) that is provided completely around one end section in an axial direction of an outer-circumferential surface of the outer ring (5a, 5b); and an engagement groove (12a, 12b) that is provided around a portion further close to a middle in the axial direction than an end section in the axial direction of the small-diameter stepped section (9a, 9b) to extend in a circumferential direction of the outer ring (5a, 5b), wherein the support hole (13a-13c) of the retainer plate (7a-7c) has an inner diameter that is larger than an outer diameter of the small-diameter stepped section (9a, 9b), and that is less than an outer diameter of a portion of the outer-circumferential surface of the outer ring (5a, 5b) that is adjacent in the axial direction to the small-diameter stepped section (9a, 9b) characterized in that the support hole (13a-13c) has large-diameter sections that are recessed outward in a radial direction of an inner-circumferential edge of the support hole (13a-13c) and that are provided at plural locations in a circumferential direction of the support hole (13a-13c), engagement protrusions (11a, 11b) are provided on parts of the circumferential edges of the large-diameter sections to protrude inward in the radial direction from the circumferential edge of the large-diameter sections, and a diameter of an inscribed circle of the engagement protrusions (11a, 11b) is less than the outer diameter of the end section in the axial direction of the small-diameter stepped section (9a, 9b), and equal to or greater than a diameter of a bottom section of the engagement groove (12a, 12b), and the outer ring (5a, 5b) and the retainer plate (7a-7c) are combined not to come apart by the engagement protrusions (11a, 11b) engaging with the engagement groove (12a, 12b) to be movable relatively to each other in the circumferential direction. an outer ring (5a, 5b) having an outer ring raceway around an inner-circumferential surface thereof; an inner ring arranged to be concentric with the outer ring (5a, 5b) and having an inner ring raceway around an outer-circumferential surface thereof; plural rolling bodies provided between the outer ring raceway and the inner ring raceway to rotate freely; and a retainer plate (7a-7c) having a support hole (13a-13c) and plural mounting holes (10, 10a, 10b) that are provided around the support hole (13a-13c), the outer ring (5a, 5b) further having: a small-diameter stepped section (9a, 9b) that is provided completely around one end section in an axial direction of an outer-circumferential surface of the outer ring (5a, 5b); and an engagement groove (12a, 12b) that is provided around a portion further close to a middle in the axial direction than an end section in the axial direction of the small-diameter stepped section (9a, 9b) to extend in a circumferential direction of the outer ring (5a, 5b), wherein the support hole (13a-13c) of the retainer plate (7a-7c) has an inner diameter that is larger than an outer diameter of the small-diameter stepped section (9a, 9b), and that is less than an outer diameter of a portion of the outer-circumferential surface of the outer ring (5a, 5b) that is adjacent in the axial direction to the small-diameter stepped section (9a, 9b) characterized in that the support hole (13a-13c) has large-diameter sections that are recessed outward in a radial direction of an inner-circumferential edge of the support hole (13a-13c) and that are provided at plural locations in a circumferential direction of the support hole (13a-13c), engagement protrusions (11a, 11b) are provided on parts of the circumferential edges of the large-diameter sections to protrude inward in the radial direction from the circumferential edge of the large-diameter sections, and a diameter of an inscribed circle of the engagement protrusions (11a, 11b) is less than the outer diameter of the end section in the axial direction of the small-diameter stepped section (9a, 9b), and equal to or greater than a diameter of a bottom section of the engagement groove (12a, 12b), and the outer ring (5a, 5b) and the retainer plate (7a-7c) are combined not to come apart by the engagement protrusions (11a, 11b) engaging with the engagement groove (12a, 12b) to be movable relatively to each other in the circumferential direction.