A hydraulic power steering system that includes: an oil pump; a steering assist device that is operated by hydraulic fluid supplied from the oil pump; a flow rate control valve that controls a flow of the hydraulic fluid that is supplied from the oil pump to the steering assist device; an electric motor that drives the flow rate control valve; a first oil passage that connects the oil pump and the flow rate control valve with each other; a second oil passage (93, 94) that connects the steering assist device and the flow rate control valve with each other; and a control unit that controls the electric motor, the flow rate control valve including a rotary valve that rotates together with the output shaft of the electric motor and a housing that has an internal space connected to the first oil passage and the second oil passage (93, 94), characterized by comprising: a rotation angle sensor that detects a rotation angle of an output shaft of the electric motor, and in that the control unit reflects an output from the rotation angle sensor on control of the electric motor; the rotation angle sensor includes a first sensor element (67, 68) that rotates together with the output shaft of the electric motor and a second sensor element (62, 63A, 63B, 64, 65, 66A, 66B) that is attached to a portion of the housing, which corresponds to the first sensor element (67, 68); the electric motor includes a stator that is attached to the housing and a rotor that is attached to a position of the output shaft of the electric motor, which corresponds to the stator; the output shaft of the electric motor has the rotary valve, the first sensor element (67, 68) and the rotor in order of the rotary valve, the first sensor element (67, 68) and the rotor along an axial direction of the output shaft or in order of the rotary valve, the rotor and the first sensor element (67, 68) along the axial direction of the output shaft; and the housing has a first accommodation chamber that accommodates the rotary valve, a second accommodation chamber that accommodates the first sensor element (67, 68) and the rotor, and an oil seal that separates the first accommodation chamber and the second accommodation chamber from each other. a rotation angle sensor that detects a rotation angle of an output shaft of the electric motor, and in that the control unit reflects an output from the rotation angle sensor on control of the electric motor; the rotation angle sensor includes a first sensor element (67, 68) that rotates together with the output shaft of the electric motor and a second sensor element (62, 63A, 63B, 64, 65, 66A, 66B) that is attached to a portion of the housing, which corresponds to the first sensor element (67, 68); the electric motor includes a stator that is attached to the housing and a rotor that is attached to a position of the output shaft of the electric motor, which corresponds to the stator; the output shaft of the electric motor has the rotary valve, the first sensor element (67, 68) and the rotor in order of the rotary valve, the first sensor element (67, 68) and the rotor along an axial direction of the output shaft or in order of the rotary valve, the rotor and the first sensor element (67, 68) along the axial direction of the output shaft; and the housing has a first accommodation chamber that accommodates the rotary valve, a second accommodation chamber that accommodates the first sensor element (67, 68) and the rotor, and an oil seal that separates the first accommodation chamber and the second accommodation chamber from each other.