A nail gun includes a contact arm that is movable relative to a nail gun body between an upper first stroke end position and a lower second stroke end position. The contact arm may prevent the nails from being driven when it is in the first stroke end position. A nail driving prevention device includes an engaging member that is movable to enter the moving path of the contact arm so as to prevent the contact arm from moving from the first stroke end position toward the second stroke end position when the number of the nails in a nail magazine has been reduced to a predetermined number. A deviating device is operable to change the moving course of the engaging member away from the moving path of the contact arm when the contact arm is positioned above the first stroke end position.