A motor-operated power steering apparatus includes a pinion shaft having a pinion, a rack shaft having a rack meshing with the pinion, a gear casing having first and second spaces therein, the pinion shaft being rotatably supported in the gear casing, the rack shaft being axially movably supported in the gear casing, an electric motor for generating assistive power, an assistive-power transmitting mechanism disposed in the gear casing for transmitting power of the electric motor to the rack shaft, and a partition member disposed between the first space in which the rack is disposed and the second space in which the assistive-power transmitting mechanism is disposed, and isolating the first and second spaces from each other for preventing dust and water from entering the assistive-power transmitting mechanism from outside. The rack shaft and the gear casing are of reduced lengths to make the power steering apparatus small in size and lightweight.