In a power steering device employing both a worm speed reducer and a backlash adjuster for a worm gear of the speed reducer, the backlash adjuster has a slider configured to be movable in a longitudinal direction of a worm shaft, and a biasing member configured to bias the slider toward one side of the longitudinal direction of the worm shaft. Also provided is a guide member configured to bias the worm shaft in a direction for meshing with a worm wheel by a movement of the worm shaft in the meshing direction that the worm shaft approaches a rotation center of the worm wheel with a movement of the slider toward the one side of the longitudinal direction, and also configured to restrict a movement of the worm shaft in a direction opposite to the meshing direction of the worm shaft.