Two guide members cooperatively constitute a chuck portion for holding a screw head. Opening and closing of the chuck portion is regulated depending on a mutual position between a machine body and a member into which a screw is driven. Guide pin portions of respective guide members engage with a stopper portion of a push lever until a screw is driven to a predetermined depth. The chuck potion tightly holds the screw head until a screw fastening operation is substantially finished, thereby suppressing the lifting of a machine body.