A nail-driving gun includes a gun body that has a handle and a nail-shooting seat. A safety device includes an upper plate, a lower plate, and a safety rod. The upper and lower plates can move synchronously between top and bottom plates of the nail-shooting seat. The safety rod has a front end that is attached fixedly to a rear end portion of the assembly of the upper and lower plates. The upper and lower plates have front ends that project forward from the nail-shooting seat and that can be pressed against a wall so as to move the safety rod to a rear limit position, where the trigger member can be actuated to shoot a nail from the nail-shooting seat.