A spindle shaft device including a shaft, a first torque sensor, and a second torque sensor. The shaft extends along an axial direction and comprises a first side portion, a second side portion, and a central portion located between the first side portion and the second side portion. The central portion has a central torsional rigidity with respect to the axial direction. The first side portion has a first torsional rigidity with respect to the axial direction. The second side portion has a second torsional rigidity with respect to the axial direction. The first torsional rigidity is smaller than the central torsional rigidity. The second torsional rigidity is smaller than the central torsional rigidity. The first torque sensor is disposed on the first side portion. The second torque sensor is disposed on the second side portion.