An electric power steering system comprising: a housing; a rack shaft that is accommodated in the housing, the rack shaft reciprocating in an axial direction in accordance with rotation of a steering shaft, and the rack shaft being formed of a material different from a material of which the housing is formed; an electric motor that is fixed to the housing and applies an assist force to the rack shaft; one rotation angle sensor that detects a rotation angle of the electric motor; a control portion that controls driving of the electric motor; and an estimate portion that estimates a temperature of the rack shaft or a characteristic value that depends on the temperature, wherein the control portion calculates a steering angle of a steering wheel based on an output of the rotation angle sensor, determines a correction amount for canceling a deviation of the calculated steering angle or the output of the rotation angle sensor, based on an estimate result obtained by the estimate portion, and corrects the calculated steering angle or the output of the rotation angle sensor by taking into account the correction amount, the deviation being caused due to a change in an ambient temperature around the rack shaft; the estimate portion is a temperature sensor disposed in a vicinity of the rack shaft, the control portion includes a storage portion in which a map is stored, the map showing a correlation between the temperature of the rack shaft and the correction amount; characterized in that the control portion determines the correction amount corresponding to the temperature at each moment, based on the output of the temperature sensor and the map stored in the storage portion, and corrects the calculated steering angle or the output of said only one rotation angle sensor by adding the correction amount to the calculated steering angle or the output of said only one rotation angle sensor. a housing; a rack shaft that is accommodated in the housing, the rack shaft reciprocating in an axial direction in accordance with rotation of a steering shaft, and the rack shaft being formed of a material different from a material of which the housing is formed; an electric motor that is fixed to the housing and applies an assist force to the rack shaft; one rotation angle sensor that detects a rotation angle of the electric motor; a control portion that controls driving of the electric motor; and an estimate portion that estimates a temperature of the rack shaft or a characteristic value that depends on the temperature, wherein the control portion calculates a steering angle of a steering wheel based on an output of the rotation angle sensor, determines a correction amount for canceling a deviation of the calculated steering angle or the output of the rotation angle sensor, based on an estimate result obtained by the estimate portion, and corrects the calculated steering angle or the output of the rotation angle sensor by taking into account the correction amount, the deviation being caused due to a change in an ambient temperature around the rack shaft; the estimate portion is a temperature sensor disposed in a vicinity of the rack shaft, the control portion includes a storage portion in which a map is stored, the map showing a correlation between the temperature of the rack shaft and the correction amount; the control portion determines the correction amount corresponding to the temperature at each moment, based on the output of the temperature sensor and the map stored in the storage portion, and corrects the calculated steering angle or the output of said only one rotation angle sensor by adding the correction amount to the calculated steering angle or the output of said only one rotation angle sensor.