An assembling method of a bearing unit which comprises a bearing having an outer ring and a retainer plate having a retainer hole for holding the outer ring, the assembling method comprising: pressing a plurality of circumferential edge portions at a circumferential edge of an inner circumferential portion of the retainer hole of the retainer plate so that the material in the inner circumferential portion is swelled out from the inner circumferential face of the retainer plate to radially inward so as to form engagement pawls protruding radially inward of the retainer hole, wherein the engagement pawls are provided in a circumferential direction on the inner circumferential portion of the retainer hole of the retainer plate; and after forming the engagement pawls, moving the engagement pawls of the retainer plate from an axial end of the outer ring to an engagement groove formed in a small diameter step portion on an outer circumferential surface and at an axial end portion of the outer ring by engaging the inner circumferential portion of the retainer hole of the retainer plate with the small diameter step portion of the outer ring such that the engagement pawl gets over the outer circumferential face from an end face of the outer ring on a side of the small diameter step portion of the outer ring to the engagement groove so as to engage the engagement pawls of the retainer plate with the engagement groove of outer ring, characterized in that the plurality of circumferential edge portions at the circumferential edge of the inner circumferential portion of the retainer hole of the retainer plate is pressed in an axial direction to form the engagement pawls such that a diameter of an inner circumferential edge of an end portion of the retainer plate on the opposite side to the pressing position is substantially the same as the diameter of the inner circumferential face before the pressing, and such that the inner circumferential edge of the end portion and a forward end of each of the engagement pawls are connected to each other by a concave surface swelling out radially inward, the forward end of each of the engagement pawls being arranged at a central portion in the axial direction of the inner circumferential portion of the retainer plate, and in that the inner circumferential portion of the retainer hole of the retainer plate is engaged with the small diameter step portion of the outer ring from the concave surface side. pressing a plurality of circumferential edge portions at a circumferential edge of an inner circumferential portion of the retainer hole of the retainer plate so that the material in the inner circumferential portion is swelled out from the inner circumferential face of the retainer plate to radially inward so as to form engagement pawls protruding radially inward of the retainer hole, wherein the engagement pawls are provided in a circumferential direction on the inner circumferential portion of the retainer hole of the retainer plate; and after forming the engagement pawls, moving the engagement pawls of the retainer plate from an axial end of the outer ring to an engagement groove formed in a small diameter step portion on an outer circumferential surface and at an axial end portion of the outer ring by engaging the inner circumferential portion of the retainer hole of the retainer plate with the small diameter step portion of the outer ring such that the engagement pawl gets over the outer circumferential face from an end face of the outer ring on a side of the small diameter step portion of the outer ring to the engagement groove so as to engage the engagement pawls of the retainer plate with the engagement groove of outer ring, characterized in that the plurality of circumferential edge portions at the circumferential edge of the inner circumferential portion of the retainer hole of the retainer plate is pressed in an axial direction to form the engagement pawls such that a diameter of an inner circumferential edge of an end portion of the retainer plate on the opposite side to the pressing position is substantially the same as the diameter of the inner circumferential face before the pressing, and such that the inner circumferential edge of the end portion and a forward end of each of the engagement pawls are connected to each other by a concave surface swelling out radially inward, the forward end of each of the engagement pawls being arranged at a central portion in the axial direction of the inner circumferential portion of the retainer plate, and in that the inner circumferential portion of the retainer hole of the retainer plate is engaged with the small diameter step portion of the outer ring from the concave surface side.