To provide a tightening and loosening assist tool for a driver that can be easily attached to a driver of a round rod-like shaft, and can tighten or loosen a screw or a nut by a strong force even without being removed from the driver even in an extremely limited space.

The assist tool for the driver includes: a head having a hole into which a rotating shaft of the driver is inserted, and a one-way clutch for transmitting torque only in one direction around the rotating shaft; and a handle for applying the torque to a head.