An electric power steering device comprising: a housing; a rotating shaft for steering disposed to be rotatable with respect to the housing and rotated by an operation of a steering wheel; a worm wheel concentrically supported with respect to the rotating shaft for steering inside the housing and configured to rotate with the rotating shaft for steering; a worm shaft having worm teeth in an axial intermediate portion, the worm teeth meshing with the worm wheel; a base end side bearing supporting a base end of the worm shaft to be rotatable with respect to the housing; a tip side bearing supporting a tip portion of the worm shaft to be rotatable with respect to the housing; an electric motor having an output shaft engaged with the worm shaft to be capable of turning force transmission; and a biasing mechanism for biasing the worm teeth toward the worm wheel by displacing the worm shaft, wherein the tip side bearing is externally fitted into and supported by the tip portion of the worm shaft, wherein an annular space is present between an outer circumferential surface of the tip side bearing and an inner circumferential surface of the housing, characterized in that the biasing mechanism includes a guide piece, a wedge piece, and an elastic member for the wedge piece, wherein the guide piece is disposed in the annular space in a state where a displacement of the guide piece in a circumferential direction of the annular space is blocked and the guide piece has a guide surface in contact, directly or via another member, with a circumferential part of the outer circumferential surface of the tip side bearing, wherein the wedge piece is disposed in the annular space in a state where displacement of the wedge piece in the circumferential direction of the annular space is allowed and the wedge piece has a wedge surface in contact, directly or via another member, with a part of the outer circumferential surface of the tip side bearing spaced apart in the circumferential direction from the circumferential part in contact with the guide surface of the guide piece, wherein the elastic member for the wedge piece applies elastic force to the wedge piece toward one circumferential side of the annular space, and wherein the biasing mechanism allows the tip side bearing to be displaced toward the worm wheel by the wedge surface and the guide surface by allowing the wedge piece to be displaced toward the one circumferential side of the annular space by the elastic force of the elastic member for the wedge piece. a housing; a rotating shaft for steering disposed to be rotatable with respect to the housing and rotated by an operation of a steering wheel; a worm wheel concentrically supported with respect to the rotating shaft for steering inside the housing and configured to rotate with the rotating shaft for steering; a worm shaft having worm teeth in an axial intermediate portion, the worm teeth meshing with the worm wheel; a base end side bearing supporting a base end of the worm shaft to be rotatable with respect to the housing; a tip side bearing supporting a tip portion of the worm shaft to be rotatable with respect to the housing; an electric motor having an output shaft engaged with the worm shaft to be capable of turning force transmission; and a biasing mechanism for biasing the worm teeth toward the worm wheel by displacing the worm shaft, wherein the tip side bearing is externally fitted into and supported by the tip portion of the worm shaft, wherein an annular space is present between an outer circumferential surface of the tip side bearing and an inner circumferential surface of the housing, characterized in that the biasing mechanism includes a guide piece, a wedge piece, and an elastic member for the wedge piece, wherein the guide piece is disposed in the annular space in a state where a displacement of the guide piece in a circumferential direction of the annular space is blocked and the guide piece has a guide surface in contact, directly or via another member, with a circumferential part of the outer circumferential surface of the tip side bearing, wherein the wedge piece is disposed in the annular space in a state where displacement of the wedge piece in the circumferential direction of the annular space is allowed and the wedge piece has a wedge surface in contact, directly or via another member, with a part of the outer circumferential surface of the tip side bearing spaced apart in the circumferential direction from the circumferential part in contact with the guide surface of the guide piece, wherein the elastic member for the wedge piece applies elastic force to the wedge piece toward one circumferential side of the annular space, and wherein the biasing mechanism allows the tip side bearing to be displaced toward the worm wheel by the wedge surface and the guide surface by allowing the wedge piece to be displaced toward the one circumferential side of the annular space by the elastic force of the elastic member for the wedge piece.