An screw tightening tool (1) includes: a handle part (22) to be held by a user; a trigger (25); a power supplying section (6, 7) that supplies an electrical power in accordance with an operated amount of the trigger; and a motor (3) that rotates in accordance with the electrical power supplied from the power supplying section. The power supplying section supplies the motor with a preventing electrical power for preventing the motor from rotating with respect to the handle part when a predetermined condition is satisfied.