A worm speed reducer comprising: a worm shaft with a first end coupled to an electric motor so as to be driven by the electric motor and a second end on the opposite side of the worm shaft in its axial direction from the first end; a worm wheel with pairs of tooth flanks (81, 82), each pair defining a tooth space; and a housing that houses the worm shaft and the worm wheel and that supports the second end of the worm shaft so as to enable the second end to pivot around the first end; wherein a plane that contains a central axis of the worm shaft and that is orthogonal to the central axis of the worm wheel is arranged at a central position of the worm wheel in a tooth width direction thereof, characterized in that the worm speed reducer further comprises a bias member supported by the housing to elastically bias the second end of the worm shaft toward the worm wheel side, and the worm wheel is formed by a cutting operation using a hob with a hob center offset from the central position in the tooth width direction, in an offset direction corresponding to a first direction along the tooth width direction, and the tooth flanks (81, 82) of each pair of the worm wheel have different pressure angles (α1, α2) at the central position in the tooth width direction. a worm shaft with a first end coupled to an electric motor so as to be driven by the electric motor and a second end on the opposite side of the worm shaft in its axial direction from the first end; a worm wheel with pairs of tooth flanks (81, 82), each pair defining a tooth space; and a housing that houses the worm shaft and the worm wheel and that supports the second end of the worm shaft so as to enable the second end to pivot around the first end; wherein a plane that contains a central axis of the worm shaft and that is orthogonal to the central axis of the worm wheel is arranged at a central position of the worm wheel in a tooth width direction thereof, characterized in that the worm speed reducer further comprises a bias member supported by the housing to elastically bias the second end of the worm shaft toward the worm wheel side, and the worm wheel is formed by a cutting operation using a hob with a hob center offset from the central position in the tooth width direction, in an offset direction corresponding to a first direction along the tooth width direction, and the tooth flanks (81, 82) of each pair of the worm wheel have different pressure angles (α1, α2) at the central position in the tooth width direction. a plane that contains a central axis of the worm shaft and that is orthogonal to the central axis of the worm wheel is arranged at a central position of the worm wheel in a tooth width direction thereof, characterized in that the worm speed reducer further comprises a bias member supported by the housing to elastically bias the second end of the worm shaft toward the worm wheel side, and the worm wheel is formed by a cutting operation using a hob with a hob center offset from the central position in the tooth width direction, in an offset direction corresponding to a first direction along the tooth width direction, and the tooth flanks (81, 82) of each pair of the worm wheel have different pressure angles (α1, α2) at the central position in the tooth width direction.