The invention provides a motor-driven power steering apparatus which can achieve a predetermined speed reduction ratio even in the case that the motor-driven power steering apparatus is constituted by a pair of spur gears or helical gears, and can secure a sufficient gear strength on the basis of a simple structure. In a motor-driven power steering apparatus in which a rotating torque of an electric motor (7) is transmitted to a steering shaft by a drive gear (6) provided in an output shaft of the electric motor (7) and a driven gear (5) provided in the steering shaft, and a speed reduction ratio is equal to or more than 3, the steering shaft and the output shaft of the electric motor (7) are arranged in almost parallel, a center distance between both the shafts is equal to or more than 35 mm and equal to or less than 90 mm, and the drive gear (6) is configured such that a number of teeth is equal to or more than 6 and equal to or less than 15, a module is equal to or more than 0.8 and equal to or less than 1.5, a tooth depth is equal to or less than 2.4 times of the module, and a pressure angle is equal to or more than 14.5 degrees and equal to or less than 30 degrees, and a torsion angle is equal to or more than 0 degrees and equal to or less than 40 degrees.