Patent ID: 8348008

Claim:
An electric power steering system comprising: a rotary shaft that rotates with a steering wheel of a vehicle; a worm speed reducing mechanism, provided at a middle portion of the rotary shaft, the worm speed reducing mechanism including a worm wheel and a worm gear meshing with the worm wheel; a mechanism housing that includes a first housing portion that covers the worm wheel and a second housing portion that covers the worm gear, the second housing portion being integrally formed with the first housing portion on a side portion of the first housing portion; an alternating-current motor that drives the worm gear, the alternating-current motor being disposed coaxially with the worm gear and fixed to an end portion of the second housing portion; a motor drive control circuit that includes a switching inverter circuit, connected to a power line of the alternating-current motor, and that controls drive of the alternating-current motor; and a control case that houses the motor drive control circuit, wherein: the mechanism housing has a control case fixation surface, on which the control case is fixed, at an end portion of the mechanism housing in an axial direction of the rotary shaft, wherein the control case fixation surface is sized so that the control case fixation surface fits in a rectangle that is the smallest rectangle that can be set in a plan view of the mechanism housing with respect to the axial direction of the rotary shaft and that has four sides, which are a pair of vertical sides that are parallel with a rotation axis of the worm gear and a pair of horizontal sides that are perpendicular to the vertical sides, in which four sides the first housing portion and the second housing portion are inscribed, the control case includes: a loop-structured, closed-loop side wall that stands on the control case fixation surface; and a ceiling wall that closes an end portion opening, on a side distant from the control case fixation surface, of the closed-loop side wall; an inverter circuit board, on which the switching inverter circuit is implemented, is fixed so as to be placed on the control case fixation surface; and a shaft insertion hole, through which the rotary shaft is passed, is formed in the ceiling wall.