An electric power steering system comprising: an electric motor disposed parallel to a rack shaft; a speed reducer including a pulley provided on a shaft portion of the electric motor and a pulley provided on a ball nut rotatably fitted to a ball screw shaft portion of the rack shaft, the pulley provided on the shaft portion and the pulley provided on the ball nut being connected to each other so that torque is transmittable between the pulleys (51, 52); a rack housing that houses the speed reducer and a main portion of the rack shaft; and a drainage device disposed below the speed reducer housed in the rack housing, wherein driving force produced by the electric motor is transmitted to the rack shaft via the speed reducer, and wherein the drainage device has a drain valve made of an elastic member, and a drain case having a case inlet that communicates with an internal space of the rack housing and a case outlet that communicates with an outside of the rack housing, wherein the drain case has a groove portion formed in a peripheral edge portion of an inner face of the case inlet, the drain valve has a main body formed in a columnar shape and fixed to the drain case, and a flange formed at one end of the main body and having an annular seal portion formed at a peripheral edge portion of the flange, when the seal portion is brought into tight contact with the peripheral edge portion of the inner face of the case inlet, the drainage device is placed in a valve closed state to form an annular space between the groove portion and the seal portion, and when the seal portion is elastically deformed to form a gap between the seal portion and the peripheral edge portion of the inner face of the case inlet, the drainage device is placed in a valve open state, characterized in that said flange is disposed so as to face the case inlet and has a bulged portion formed at a center portion of the flange and bulged toward the case inlet, wherein: a bottom face of the drain case has a protrusion protruded toward the case inlet, and the other end of the main body of the drain valve has a recessed portion in which the protrusion is fitted; and the drain valve is fixed to the drain case by fitting the protrusion in the recessed portion. an electric motor disposed parallel to a rack shaft; a speed reducer including a pulley provided on a shaft portion of the electric motor and a pulley provided on a ball nut rotatably fitted to a ball screw shaft portion of the rack shaft, the pulley provided on the shaft portion and the pulley provided on the ball nut being connected to each other so that torque is transmittable between the pulleys (51, 52); a rack housing that houses the speed reducer and a main portion of the rack shaft; and a drainage device disposed below the speed reducer housed in the rack housing, wherein driving force produced by the electric motor is transmitted to the rack shaft via the speed reducer, and wherein the drainage device has a drain valve made of an elastic member, and a drain case having a case inlet that communicates with an internal space of the rack housing and a case outlet that communicates with an outside of the rack housing, wherein the drain case has a groove portion formed in a peripheral edge portion of an inner face of the case inlet, the drain valve has a main body formed in a columnar shape and fixed to the drain case, and a flange formed at one end of the main body and having an annular seal portion formed at a peripheral edge portion of the flange, when the seal portion is brought into tight contact with the peripheral edge portion of the inner face of the case inlet, the drainage device is placed in a valve closed state to form an annular space between the groove portion and the seal portion, and when the seal portion is elastically deformed to form a gap between the seal portion and the peripheral edge portion of the inner face of the case inlet, the drainage device is placed in a valve open state, characterized in that said flange is disposed so as to face the case inlet and has a bulged portion formed at a center portion of the flange and bulged toward the case inlet, wherein: a bottom face of the drain case has a protrusion protruded toward the case inlet, and the other end of the main body of the drain valve has a recessed portion in which the protrusion is fitted; and the drain valve is fixed to the drain case by fitting the protrusion in the recessed portion. a bottom face of the drain case has a protrusion protruded toward the case inlet, and the other end of the main body of the drain valve has a recessed portion in which the protrusion is fitted; and the drain valve is fixed to the drain case by fitting the protrusion in the recessed portion.