A ball bearing for rotatably supporting a steering shaft, which is comprised of an input shaft and an output shaft connected to each other via a torsion bar is secured by a lock nut. The lock nut has an inside diameter Y1 greater than a maximum outside diameter X6 of the sensor body of a torque sensor mainly configured from a permanent magnet, first and second yoke members, and a magnetic sensor, which are disposed on the outer periphery of the steering shaft. With this configuration, the torque sensor and the lock nut radially overlap with each other, thus reducing the size of a device. Also, because of the lock nut being assembled so as to pass through the outer peripheral side of the torque sensor, component parts can be installed from one direction, thus improving the assembling workability of the device.