A wheel, comprising: a barrel holder rotationally driven; a plurality of barrel support shafts arranged on a circumference of the barrel holder at an angle to a rotation axis of the barrel holder; barrels rotatably supported on the corresponding barrel support shafts, each of the barrels being a barrel-shaped rotating body, outer surfaces of the barrels sequentially contacting a surface on which the wheel travels as the barrel holder rotates; and bearings disposed between the respective barrel support shafts and the respective barrels, each of the bearings rotatably supporting the corresponding barrel and having a retainer, wherein the bearing is disposed at an axially outer end of the barrel and encloses the barrel support shaft, wherein a groove is provided at least in a region of an outer circumferential surface of the barrel support shaft, the region extends in an axial direction and includes a radially inner projected area of the retainer, wherein the bearing includes: a first race abutting a first member in the axial direction, the first member being disposed closer to the barrel than the bearing; a second race abutting a second member in the axial direction, the second member being disposed closer to the barrel holder than the bearing; and a plurality of rolling elements disposed between the first race and the second race, the plurality of rolling elements rollably contacting the first race and the second race; characterized in that the retainer is disposed between the first race and the second race and supports the plurality of rolling elements, the groove is provided in the outer circumferential surface of the barrel support shaft, the groove extends in the axial direction and crosses over an end portion of an inner end surface of the first race and an end portion of an inner end surface of the second race, and wherein the inner end surface of the first race and the inner end surface of the second race oppose to each other. a barrel holder rotationally driven; a plurality of barrel support shafts arranged on a circumference of the barrel holder at an angle to a rotation axis of the barrel holder; barrels rotatably supported on the corresponding barrel support shafts, each of the barrels being a barrel-shaped rotating body, outer surfaces of the barrels sequentially contacting a surface on which the wheel travels as the barrel holder rotates; and bearings disposed between the respective barrel support shafts and the respective barrels, each of the bearings rotatably supporting the corresponding barrel and having a retainer, wherein the bearing is disposed at an axially outer end of the barrel and encloses the barrel support shaft, wherein a groove is provided at least in a region of an outer circumferential surface of the barrel support shaft, the region extends in an axial direction and includes a radially inner projected area of the retainer, wherein the bearing includes: a first race abutting a first member in the axial direction, the first member being disposed closer to the barrel than the bearing; a second race abutting a second member in the axial direction, the second member being disposed closer to the barrel holder than the bearing; and a plurality of rolling elements disposed between the first race and the second race, the plurality of rolling elements rollably contacting the first race and the second race; characterized in that the retainer is disposed between the first race and the second race and supports the plurality of rolling elements, the groove is provided in the outer circumferential surface of the barrel support shaft, the groove extends in the axial direction and crosses over an end portion of an inner end surface of the first race and an end portion of an inner end surface of the second race, and wherein the inner end surface of the first race and the inner end surface of the second race oppose to each other. a first race abutting a first member in the axial direction, the first member being disposed closer to the barrel than the bearing; a second race abutting a second member in the axial direction, the second member being disposed closer to the barrel holder than the bearing; and a plurality of rolling elements disposed between the first race and the second race, the plurality of rolling elements rollably contacting the first race and the second race; characterized in that the retainer is disposed between the first race and the second race and supports the plurality of rolling elements, the groove is provided in the outer circumferential surface of the barrel support shaft, the groove extends in the axial direction and crosses over an end portion of an inner end surface of the first race and an end portion of an inner end surface of the second race, and wherein the inner end surface of the first race and the inner end surface of the second race oppose to each other.