An electric power steering system includes a rack housing configured to accommodate a rack shaft, a pinion shaft meshed with the rack shaft, a speed-reducer including a worm wheel connected to a portion of the second pinion shaft protruding from the rack housing, a speed-reducer housing configured to accommodate the speed-reducer, and a seal member having an annular shape. The seal member is disposed closer to the second pinion gear than the worm wheel is, in the speed-reducer housing. The sear member seals a clearance between the second pinion shaft and the speed-reducer housing.