An electric power steering device includes a rack bar; a pinion shaft meshing with the rack bar; a worm wheel provided on the pinion shaft; a worm shaft meshing with the worm wheel; an electric motor provided on the worm shaft; a first housing accommodating at least a part of the rack bar and the pinion shaft and includes a first bolt boss formed with a through-hole; a second housing; and a bolt. The second housing includes a worm-wheel accommodating portion accommodating the worm wheel, and a second bolt boss provided to have a shorter distance to the first housing than that of the worm-wheel accommodating portion, and formed with a female-threaded portion. The second bolt boss is opposed to the first bolt boss to cause the female-threaded portion to be continuous with the through-hole of the first bolt boss. The bolt fastens the first housing to the second housing.