A worm reduction gear comprising: a housing with a holding hole formed therein; a worm shaft including a first end coupled to an electric motor and a second end positioned on the opposite side of the worm shaft from the first end in an axial direction, the worm shaft being housed in the housing; a worm wheel that meshes with the worm shaft; a first bearing held by the housing to support the first end so that the first end is rotatable; a second bearing that supports the second end so that the second end is rotatable; a bias member that directly or indirectly biases the second end toward the worm wheel; and a guide member formed of a resin material and having a slit, wherein the guide member is elastically fitted in the holding hole in the housing to directly or indirectly guide movement of the second end, the guide member includes a C-shaped fitting portion that has a pair of circumferential ends between which the slit is formed, and that is elastically fitted in the holding hole, the fitting portion has an outer surface, an inner surface, a first end face, and a second end face positioned on the opposite side of the fitting portion from the first end face in the axial direction, the inner surface defines a guide hole serving as a guide space through which the second bearing is guided, the guide hole being formed like a bias hole in which the second bearing is held such that the second end of the worm shaft is movable in a first direction and in a second direction, the guide hole being open in the second direction via the slit, and the first direction is a direction in which a distance between a central axis of the worm shaft and a central axis of the worm wheel increases, and the second direction is opposite to the first direction, characterized in that the inner surface that defines the guide hole extends from the first end face to the second end face. a housing with a holding hole formed therein; a worm shaft including a first end coupled to an electric motor and a second end positioned on the opposite side of the worm shaft from the first end in an axial direction, the worm shaft being housed in the housing; a worm wheel that meshes with the worm shaft; a first bearing held by the housing to support the first end so that the first end is rotatable; a second bearing that supports the second end so that the second end is rotatable; a bias member that directly or indirectly biases the second end toward the worm wheel; and a guide member formed of a resin material and having a slit, wherein the guide member is elastically fitted in the holding hole in the housing to directly or indirectly guide movement of the second end, the guide member includes a C-shaped fitting portion that has a pair of circumferential ends between which the slit is formed, and that is elastically fitted in the holding hole, the fitting portion has an outer surface, an inner surface, a first end face, and a second end face positioned on the opposite side of the fitting portion from the first end face in the axial direction, the inner surface defines a guide hole serving as a guide space through which the second bearing is guided, the guide hole being formed like a bias hole in which the second bearing is held such that the second end of the worm shaft is movable in a first direction and in a second direction, the guide hole being open in the second direction via the slit, and the first direction is a direction in which a distance between a central axis of the worm shaft and a central axis of the worm wheel increases, and the second direction is opposite to the first direction, characterized in that the inner surface that defines the guide hole extends from the first end face to the second end face.