An electric power steering device comprising: a worm wheel that is provided on a steering shaft linked to a steering wheel; a worm shaft that meshes with the worm wheel; an electric motor that rotates the worm shaft; a first bearing that rotatably supports a base end side of the worm shaft; a second bearing that rotatably supports a tip end side of the worm shaft; a biasing member configured to bias the worm shaft toward the worm wheel via the second bearing; and a shaft coupler configured to couple the worm shaft and an output shaft of the electric motor so that the worm shaft is capable of tilting with respect to the output shaft of the electric motor by using the first bearing as a fulcrum; the shaft coupler comprising: a joint on a motor side that is connected to the output shaft of the electric motor; a joint on a worm side that is connected to the worm shaft; and a coupling that is provided between the joint on the motor side and the joint on the worm side and that is for transmitting rotational torque of the electric motor to the worm shaft, wherein the joint on the motor side and the joint on the worm side, each having a plurality of protrusion parts (32, 42) that extend in an axial direction, are arranged so that the protrusion parts (32, 42) are aligned alternately in a circumferential direction, characterized in that the coupling is formed by a plurality of independent caps that are fitted to the protrusion parts (32, 42) of the joint on the motor side or the joint on the worm side, wherein the joint on the motor side and the joint on the worm side are equal in shape, each of the caps has a ring shape that is fitted to an outer peripheral surface of each of the protrusion parts (32, 42), each of the caps comprises a pair of torque transmission parts that is sandwiched between the protrusion parts (32, 42), being neighboring to each other, of the joint on the motor side and the joint on the worm side and that transmits the rotational torque of the electric motor to the worm shaft, outer surfaces of the torque transmission parts are in contact with the either one of the protrusion parts (32, 42) that does not fit in the caps. a worm wheel that is provided on a steering shaft linked to a steering wheel; a worm shaft that meshes with the worm wheel; an electric motor that rotates the worm shaft; a first bearing that rotatably supports a base end side of the worm shaft; a second bearing that rotatably supports a tip end side of the worm shaft; a biasing member configured to bias the worm shaft toward the worm wheel via the second bearing; and a shaft coupler configured to couple the worm shaft and an output shaft of the electric motor so that the worm shaft is capable of tilting with respect to the output shaft of the electric motor by using the first bearing as a fulcrum; a joint on a motor side that is connected to the output shaft of the electric motor; a joint on a worm side that is connected to the worm shaft; and a coupling that is provided between the joint on the motor side and the joint on the worm side and that is for transmitting rotational torque of the electric motor to the worm shaft, wherein the joint on the motor side and the joint on the worm side, each having a plurality of protrusion parts (32, 42) that extend in an axial direction, are arranged so that the protrusion parts (32, 42) are aligned alternately in a circumferential direction, characterized in that the coupling is formed by a plurality of independent caps that are fitted to the protrusion parts (32, 42) of the joint on the motor side or the joint on the worm side, wherein the joint on the motor side and the joint on the worm side are equal in shape, each of the caps has a ring shape that is fitted to an outer peripheral surface of each of the protrusion parts (32, 42), each of the caps comprises a pair of torque transmission parts that is sandwiched between the protrusion parts (32, 42), being neighboring to each other, of the joint on the motor side and the joint on the worm side and that transmits the rotational torque of the electric motor to the worm shaft, outer surfaces of the torque transmission parts are in contact with the either one of the protrusion parts (32, 42) that does not fit in the caps.