A worm reduction gear comprising: a housing; a rotation shaft which is supported to be rotatable with respect to the housing; a worm wheel (4a-4i) which has a worm wheel tooth part in an outer circumferential surface and is externally fitted and fixed to the rotation shaft; and a worm shaft which has a worm tooth part in an axial intermediate portion of an outer circumferential surface and is supported to be rotatable with respect to the housing in a state where the worm tooth part is engaged with the worm wheel tooth part, wherein: the worm wheel (4a-4i) includes: an inner wheel element (15a-15i); and an outer wheel element (16a-16i); the inner wheel element (15a-15i) has a first annular concave part (22, 22a, 22b) provided to be recessed in an axial direction in an axial one-side surface; and characterized in that: the outer circumferential surface of the inner wheel element (15a-15i) configuring the worm wheel (4a-4i) is formed to be a cylindrical surface part (24, 24a) in an axial range which is radially superimposed with an entire engaging portion of the worm wheel tooth part and the worm tooth part; the outer wheel element (16a-16i) is made of a synthetic resin, and has the worm wheel tooth part in an outer circumferential surface, a second annular concave part (38, 38a-38c) is provided to be recessed in the axial direction in the axial other-side surface of the inner wheel element (15a-15i); and the inner wheel element (15a-15i) is embedded in the outer wheel element (16a-16i), such that a continuous range from the inner diameter side circumferential surface configuring the inner surface of the first annular concave part (22, 22a, 22b) through the outer circumferential surface of the inner wheel element (15a-15i) to an inner diameter side circumferential surface configuring an inner surface of the second annular concave part (38, 38a-38c) in the surface of the inner wheel element (15a-15i) is covered over an entire circumference. a housing; a rotation shaft which is supported to be rotatable with respect to the housing; a worm wheel (4a-4i) which has a worm wheel tooth part in an outer circumferential surface and is externally fitted and fixed to the rotation shaft; and a worm shaft which has a worm tooth part in an axial intermediate portion of an outer circumferential surface and is supported to be rotatable with respect to the housing in a state where the worm tooth part is engaged with the worm wheel tooth part, wherein: the worm wheel (4a-4i) includes: an inner wheel element (15a-15i); and an outer wheel element (16a-16i); an inner wheel element (15a-15i); and an outer wheel element (16a-16i); the inner wheel element (15a-15i) has a first annular concave part (22, 22a, 22b) provided to be recessed in an axial direction in an axial one-side surface; and characterized in that: the outer circumferential surface of the inner wheel element (15a-15i) configuring the worm wheel (4a-4i) is formed to be a cylindrical surface part (24, 24a) in an axial range which is radially superimposed with an entire engaging portion of the worm wheel tooth part and the worm tooth part; the outer wheel element (16a-16i) is made of a synthetic resin, and has the worm wheel tooth part in an outer circumferential surface, a second annular concave part (38, 38a-38c) is provided to be recessed in the axial direction in the axial other-side surface of the inner wheel element (15a-15i); and the inner wheel element (15a-15i) is embedded in the outer wheel element (16a-16i), such that a continuous range from the inner diameter side circumferential surface configuring the inner surface of the first annular concave part (22, 22a, 22b) through the outer circumferential surface of the inner wheel element (15a-15i) to an inner diameter side circumferential surface configuring an inner surface of the second annular concave part (38, 38a-38c) in the surface of the inner wheel element (15a-15i) is covered over an entire circumference. the outer circumferential surface of the inner wheel element (15a-15i) configuring the worm wheel (4a-4i) is formed to be a cylindrical surface part (24, 24a) in an axial range which is radially superimposed with an entire engaging portion of the worm wheel tooth part and the worm tooth part; the outer wheel element (16a-16i) is made of a synthetic resin, and has the worm wheel tooth part in an outer circumferential surface, a second annular concave part (38, 38a-38c) is provided to be recessed in the axial direction in the axial other-side surface of the inner wheel element (15a-15i); and the inner wheel element (15a-15i) is embedded in the outer wheel element (16a-16i), such that a continuous range from the inner diameter side circumferential surface configuring the inner surface of the first annular concave part (22, 22a, 22b) through the outer circumferential surface of the inner wheel element (15a-15i) to an inner diameter side circumferential surface configuring an inner surface of the second annular concave part (38, 38a-38c) in the surface of the inner wheel element (15a-15i) is covered over an entire circumference.