To provide a bolt fastening device as well as a method for fastening a bolt capable of identifying bolt abrasion and an improper bolt fastening state.

A sensor is provided to detect an annular projection formed on the operation rod 18 of a power tool used for fastening a bolt. Also, the proper or improper bolt fastening state is identified, using a torque change detected in the motor 16 of the power tool and a signal detected with the sensor. The reliability of a bolt fastening result is improved by accurately identifying a bolt fastening state for a fastened object, and productivity in a succeeding process is thereby improved. At the same time, the nonconformity of a product is minimized.