A planet gear train based torque detector is provided with idler gears between an input shaft and an output shaft wherein a rotation of the input shaft rotates the idler gears which rotate the output shaft, and a force is applied to a tangent line between each idler gear and the input shaft; peripheral resilient members, each having a fixed first end and a second end for receiving force applied thereon by the revolution of idler gears; and at least one strain gauge, each attached on one resilient member for measuring a deformation at the resilient member due to the revolution of the idler gears. A torque between the input shaft and the output shaft is obtained by calculating the deformation. The resilient members parallel the input shaft.