Disclosed herein is a nailer having an improved safety device. With the improved safety device, a movable plate, which is adapted to slidably move along a lower surface of a guiding unit, is formed, at a certain location thereof, with an insertion hole for the insertion of a protrusion formed at a trigger. Thereby, if the trigger is pulled in a pressed state of the movable plate, a nailing operation is accomplished. Also, in the course of pulling the trigger, the movable plate can be further inserted rearward by a predetermined distance under interaction of the insertion hole and protrusion. This has the effect of eliminating the risk of causing scratches on a nailing surface of a target structure even if the movable plate is moved while coming into close contact, at a front end thereof, with the nailing surface, resulting in several advantages, such as for example, easy nailing operation, operator's convenient working posture, and rapid and safe nailing operation.