A worm reducer comprising: a housing (12a, 12b); a worm wheel which is supported on a driven shaft rotatably supported by the housing to be concentric with the driven shaft and rotates together with the driven shaft; and a worm, wherein in a state where an axial one end portion is joined with a drive shaft provided at a skew position with respect to the driven shaft, and worm teeth provided on an axial intermediate portion are engaged with the worm wheel, an axial one side portion with respect to a portion where the worm teeth are provided is rotatably supported by a bearing with respect to the housing (12a, 12b), wherein the bearing (18c, 18e) is a rolling bearing which includes: an outer ring (19a, 19b) having an outer ring raceway on an inner circumferential surface; an inner ring having an inner ring raceway on an outer circumferential surface; and a plurality of rolling elements rotatably arranged between the outer ring raceway and the inner ring raceway, an outer circumferential surface of an axial one end portion of the outer ring (19a, 19b) is provided with a flange portion which protrudes radially outward, the housing (12a, 12b) is formed with a holding portion (20a, 20b) to be internally fitted with the outer ring (19a, 19b), in a state where an axial another side portion of the outer ring (19a, 19b) is internally fitted in an axial another side portion of the holding portion (20a, 20b) and an axial another side face of the flange portion is abutted against an axial one side face of the housing (12a, 12b), an axial another side face of a retaining plate supported to the housing (12a, 12b) is abutted against an axial one side face of the flange portion, the flange portion is arranged at an entire circumference or at intervals in a circumferential direction on the outer circumferential surface of the axial one end portion of the outer ring (19a, 19b), an axial one end portion of the holding portion (20a, 20b) is provided with a large diameter stepped part (31, 31a), an inner diameter of which is larger than that of an axial another side portion, at an entire circumference or at intervals in the circumferential direction, characterized in that a stepped surface at an axial another end portion of the large diameter stepped part (31, 31a) defines the axial one side face of the housing (12a, 12b), and wherein in a state where the axial another side of the outer ring (19a, 19b) is internally fitted in the axial another side portion of the holding portion (20a, 20b) and the axial another side face of the flange portion is abutted against the stepped surface, an axial another side face of the retaining plate supported by the housing is abutted against the axial one side face of the flange portion, and the flange portion is axially sandwiched between the stepped surface and the retaining plate. a housing (12a, 12b); a worm wheel which is supported on a driven shaft rotatably supported by the housing to be concentric with the driven shaft and rotates together with the driven shaft; and a worm, wherein in a state where an axial one end portion is joined with a drive shaft provided at a skew position with respect to the driven shaft, and worm teeth provided on an axial intermediate portion are engaged with the worm wheel, an axial one side portion with respect to a portion where the worm teeth are provided is rotatably supported by a bearing with respect to the housing (12a, 12b), wherein the bearing (18c, 18e) is a rolling bearing which includes: an outer ring (19a, 19b) having an outer ring raceway on an inner circumferential surface; an inner ring having an inner ring raceway on an outer circumferential surface; and a plurality of rolling elements rotatably arranged between the outer ring raceway and the inner ring raceway, an outer circumferential surface of an axial one end portion of the outer ring (19a, 19b) is provided with a flange portion which protrudes radially outward, the housing (12a, 12b) is formed with a holding portion (20a, 20b) to be internally fitted with the outer ring (19a, 19b), in a state where an axial another side portion of the outer ring (19a, 19b) is internally fitted in an axial another side portion of the holding portion (20a, 20b) and an axial another side face of the flange portion is abutted against an axial one side face of the housing (12a, 12b), an axial another side face of a retaining plate supported to the housing (12a, 12b) is abutted against an axial one side face of the flange portion, the flange portion is arranged at an entire circumference or at intervals in a circumferential direction on the outer circumferential surface of the axial one end portion of the outer ring (19a, 19b), an axial one end portion of the holding portion (20a, 20b) is provided with a large diameter stepped part (31, 31a), an inner diameter of which is larger than that of an axial another side portion, at an entire circumference or at intervals in the circumferential direction, characterized in that a stepped surface at an axial another end portion of the large diameter stepped part (31, 31a) defines the axial one side face of the housing (12a, 12b), and wherein in a state where the axial another side of the outer ring (19a, 19b) is internally fitted in the axial another side portion of the holding portion (20a, 20b) and the axial another side face of the flange portion is abutted against the stepped surface, an axial another side face of the retaining plate supported by the housing is abutted against the axial one side face of the flange portion, and the flange portion is axially sandwiched between the stepped surface and the retaining plate.