PURPOSE: To reduce the number of constituent part items for making a power transmission device compact, reduce cost, and improve torque detection accuracy by arranging an input detection means and a resultant force section on the same axis between an input force input section and a drive section.

CONSTITUTION: A rotary potentiometer (input detection section) 30 for detecting torsional angle (phase difference) of both of a drive shaft (input force input section) 24 and a resultant force shaft (resultant force section) 25 is provided on them. The drive shaft 24, the resultant force shaft 25, a torsion bar 29, and the rotary potentiometer 30 are arranged on the same axis. Consequently, it is possible to simplify configurations of a torque detection system and a power transmission system, reduce the number of constituent part items, make a power transmission device 11 compact, and reduce cost. Moreover, it is possible to solve the problems of accumulation of dimensional tolerance of parts and large friction and ensure high torque detection accuracy.