A safety device for a nailer has a movable element movable between a locking condition and a release firing condition. At least one lock has a stopping end movable to engage an abutment fixed to or carried by the movable element, for stopping it in the locking condition. When disengaged, the stopping end allows translation to the release position. The lock is also provided with an elastic element which acts in the direction of the stopping condition, with an arming mechanism acting counter to the elastic element, for shifting the lock and the respective stopping end for moving to the armed condition. An operator, to put the nailer in the armed and released conditions, must act on the arming mechanism to free the abutment from the stopping end, and he must place the movable element in the release condition to shoot the nail by pulling a trigger.