A driver torque estimation device comprising: a transfer ratio variation device and a torsion bar provided on a torque transfer path between a steering wheel and a steering mechanism; an electric motor configured to apply a steering assist force to the steering mechanism; a torque sensor that acquires torsion bar torque applied to the torsion bar; an input shaft rotational angle acquisition unit configured to acquire a rotational angle of an input shaft of the transfer ratio variation device;and an electronic control unit configured to acquire a rotational angle of an output shaft of the transfer ratio variation device, the driver torque estimation device being characterized by the electronic control unit being configured to compute an estimated value of first disturbance due to a first electric power steering constituent member, which is positioned on an input shaft side of the transfer ratio variation device, using the rotational angle of the input shaft, the first disturbance acting on at least one of the steering wheel and the input shaft between the steering wheel and the transfer ratio variation device; the electronic control unit is configured to compute an estimated value of second disturbance due to a second electric power steering constituent member, which is positioned on an output shaft side of the transfer ratio variation device, using the rotational angle of the output shaft, the second disturbance acting on a rotary shaft between the transfer ratio variation device and the torsion bar; and the electronic control unit is configured to compute an estimated value of driver torque based on the torsion bar torque, the estimated value of the first disturbance, and the estimated value of the second disturbance. a transfer ratio variation device and a torsion bar provided on a torque transfer path between a steering wheel and a steering mechanism; an electric motor configured to apply a steering assist force to the steering mechanism; a torque sensor that acquires torsion bar torque applied to the torsion bar; an input shaft rotational angle acquisition unit configured to acquire a rotational angle of an input shaft of the transfer ratio variation device;and an electronic control unit configured to acquire a rotational angle of an output shaft of the transfer ratio variation device, the driver torque estimation device being characterized by the electronic control unit being configured to compute an estimated value of first disturbance due to a first electric power steering constituent member, which is positioned on an input shaft side of the transfer ratio variation device, using the rotational angle of the input shaft, the first disturbance acting on at least one of the steering wheel and the input shaft between the steering wheel and the transfer ratio variation device; the electronic control unit is configured to compute an estimated value of second disturbance due to a second electric power steering constituent member, which is positioned on an output shaft side of the transfer ratio variation device, using the rotational angle of the output shaft, the second disturbance acting on a rotary shaft between the transfer ratio variation device and the torsion bar; and the electronic control unit is configured to compute an estimated value of driver torque based on the torsion bar torque, the estimated value of the first disturbance, and the estimated value of the second disturbance.