Protection mechanism for a band saw machine

A protective device of a bandsaw machine comprises a tension adjusting device comprising a movable base, which is connected to a driven wheel for adjusting tension of a saw blade, which runs around the driven wheel. An actuation disk is connected to the movable base and is arranged above a relay, so that the operation plate can be moved together with the movable base to turn on or the relay off, thereby controlling the operation of a motor is controlled electrically connected to the relay is. When the blade is changed by moving the movable base, the engine will therefore can no longer work because of the relay which is turned off by the operation of the operation panel to assure safety of a user.