An electric assist bicycle capable of traveling with a combination of a human driving force generated by a pedal force from a pedal and an auxiliary driving force generated by a motor provided in a motor drive unit disposed at an intermediate position between a front wheel and a rear wheel, the electric assist bicycle being configured such that a cylindrical human power transmission member for receiving a transmitted human driving force is disposed on an outer periphery of a crank shaft for receiving the human driving force from the pedal, an interlocking member is disposed on the outer periphery of the crank shaft so as to receive, from one end of the interlocking member, the human driving force transmitted from the human power transmission member and includes a driving force output wheel attached to another end of the interlocking member so as to be coaxial with the crank shaft, the interlocking member combining, in the motor driving unit, the human driving force and an auxiliary driving force transmitted in the motor drive unit from the motor, and a driving force from the interlocking member is transmitted to the rear wheel through the driving force output wheel and an endless driving force transmission member looped over the driving force output wheel, the electric assist bicycle comprising: a deceleration mechanism on an auxiliary-driving-force transmission path from the motor to the interlocking member, the deceleration mechanism including a reduction gear and a reduction gear support shaft (36a, 36b) that supports the reduction gear, a one-way clutch for interrupting a human driving force, the one-way clutch being disposed between the reduction gear and the reduction gear support shaft (36a, 36b) so as to prevent a human driving force transmitted in the motor drive unit from the interlocking member from being transmitted to the motor, the human power transmission member being engaged with an intermediate cylinder on the outer periphery of the crank shaft with a one-way clutch for interrupting an auxiliary driving force between the intermediate cylinder and the interlocking member, characterized in that the human power transmission member has a magneto-striction generation portion for a torque sensor for detecting the human driving force, and the electric assist bicycle further comprises a rotation detector that detects a rotation of one of the intermediate cylinder and the human power transmission member, the rotation detector being an optical sensor. a deceleration mechanism on an auxiliary-driving-force transmission path from the motor to the interlocking member, the deceleration mechanism including a reduction gear and a reduction gear support shaft (36a, 36b) that supports the reduction gear, a one-way clutch for interrupting a human driving force, the one-way clutch being disposed between the reduction gear and the reduction gear support shaft (36a, 36b) so as to prevent a human driving force transmitted in the motor drive unit from the interlocking member from being transmitted to the motor, the human power transmission member being engaged with an intermediate cylinder on the outer periphery of the crank shaft with a one-way clutch for interrupting an auxiliary driving force between the intermediate cylinder and the interlocking member, characterized in that the human power transmission member has a magneto-striction generation portion for a torque sensor for detecting the human driving force, and the electric assist bicycle further comprises a rotation detector that detects a rotation of one of the intermediate cylinder and the human power transmission member, the rotation detector being an optical sensor.