Screw driver

A screw driver comprising;

• a housing;
• a drive portion having an output shaft rotatable in forward direction and reverse direction;
• an end bit mounting portion that mounts an end bit, the end bit mounting portion being drivingly rotatable in forward direction for fastening a screw and in reverse direction for unfastening a screw, and movable between a frontward position and a rearward position;
• a clutch mechanism disposed between the driving portion and the end bit mounting portion for transmitting driving force from the drive portion to the end bit mounting portion in the forward and reverse directions when the end bit mounting portion is at the rearward position;
• and,
• a co-rotation preventing mechanism assembled in the housing, the co-rotation preventing mechanism being engageable with the end bit mounting portion when the end bit mounting portion is at the frontward position wherein said co-rotation preventing mechanism includes a portion that is rotatable only in the reverse direction 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, and being 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.