A reduction gear unit that accommodates, in a gear case, a pinion that transmits a rotational force, a large gear that meshes with the pinion and transmits a rotational force, and a bearing that is located on each of both sides of the pinion and is configured to include an inner race, an outer race, and a plurality of rollers, wherein each of the rollers is configured to have a shape tapered from a large-diameter-side end surface provided on a side of the pinion toward a small-diameter-side end surface provided on an opposite side to the side of the pinion, the inner race includes a small-diameter-side flange portion that is provided on a part of the small-diameter-side end surface of the roller, which is opposite to the pinion, and extends in a direction from the inner race to the outer race, and a large-diameter-side flange portion that extends in a direction from the inner race to the outer race while enclosing the large-diameter-side end surface of the roller, so as to be close to the outer race and forms an opening between the outer race and the large-diameter-side flange portion, characterized in that: a ring (50; 51) that has an annular shape, and that is configured to include an inner peripheral surface (50c; 51c) that surrounds an outer peripheral surface of a pinion shaft provided through the pinion, a gear-side end surface (50e; 51e) that is provided on the side of the pinion, a bearing-side end surface (50b; 51b) that is provided on a side of the bearing, and an outer peripheral surface (50d; 51d) that is inclined from the gear-side end surface (50e; 51e) toward an outer peripheral edge (50a; 51a) of the bearing-side end surface (50b; 51b), is provided between the pinion and the inner race, and when a length from an outer peripheral edge of the gear-side end surface (50e; 51e) to an axial line of the pinion shaft is represented as L10, a length from a tooth root of the large gear to the axial line is represented as L11, a length from an outer peripheral edge (50a; 51a) of a gear-side end surface of the large-diameter-side flange portion to the axial line is represented as L2, and a length from an outer peripheral edge (50a; 51a) of the bearing-side end surface (50b; 51b) to the axial line is represented as L3, the outer peripheral surface (50d; 51d) of the ring (50; 51) has a shape satisfying a relationship expressed as L10<L11, and also satisfying a relationship expressed as L3≥L2. a ring (50; 51) that has an annular shape, and that is configured to include an inner peripheral surface (50c; 51c) that surrounds an outer peripheral surface of a pinion shaft provided through the pinion, a gear-side end surface (50e; 51e) that is provided on the side of the pinion, a bearing-side end surface (50b; 51b) that is provided on a side of the bearing, and an outer peripheral surface (50d; 51d) that is inclined from the gear-side end surface (50e; 51e) toward an outer peripheral edge (50a; 51a) of the bearing-side end surface (50b; 51b), is provided between the pinion and the inner race, and when a length from an outer peripheral edge of the gear-side end surface (50e; 51e) to an axial line of the pinion shaft is represented as L10, a length from a tooth root of the large gear to the axial line is represented as L11, a length from an outer peripheral edge (50a; 51a) of a gear-side end surface of the large-diameter-side flange portion to the axial line is represented as L2, and a length from an outer peripheral edge (50a; 51a) of the bearing-side end surface (50b; 51b) to the axial line is represented as L3, the outer peripheral surface (50d; 51d) of the ring (50; 51) has a shape satisfying a relationship expressed as L10<L11, and also satisfying a relationship expressed as L3≥L2.