A screw driver capable of avoiding co-rotation of an end bit and a motor in case of sharp shooting the end bit with a groove in a head of a screw. A co-rotation preventing mechanism is engageable with an end bit mounting portion when the end bit mounting portion is at a frontward position to prevent the end bit mounting portion from being rotated in the forward direction and to allow the end bit mounting portion to rotate in the reverse direction. The preventing mechanism is disengageable from the end bit mounting portion when the end bit mounting portion is at the rearward position to allow the end bit mounting portion to rotate in both forward and reverse directions.