A built-in step is conducted by building a drive part which generates a rotation drive power, a rotation shaft to be rotated by the rotation drive power of the drive part through a gear mechanism, a lock stopper to be screwed with the rotation shaft to move axially by rotating of the rotation shaft, a lock bar which moves between a lock position for locking a steering shaft by movement of the lock stopper and an unlock position for unlocking the steering shaft, and a first spring interposed between the lock stopper and the lock bar for giving a bias load, into a lock body to be installed in a mounting hole part of a steering column post of a vehicle from the same direction. Thereafter, a screwing step for screwing the rotation shaft and the lock stopper after the built-in step is conducted to assemble an electric steering lock device.