A torque detecting device and an electric power steering system are provided. A torque sensor includes a first rotor that is fixed to an input shaft and that has a predetermined shaft angle multiplier, and a second rotor that is fixed to an output shaft and that has a shaft angle multiplier different from that of the first rotor. In first to fourth output coils arranged on a stator that is arranged around outer peripheries of the first and second rotors, electric signals having different phases are generated on the basis of a variation of a gap between the stator and the first and second rotors. A twist angle of a torsion bar is calculated on the basis of the electric signals, and a torque that acts between the input shaft and the output shaft is calculated on the basis of the twist angle.
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