A handgun enables the adapter plate for mounting the dot-sight to be securely and easily assembled into the slide even without using a fastening member such as a screw. The handgun equipes with the adapter plate and the slide for mounting the dot-sight with the improved assembly structure, wherein the adapter plate slides toward one side and is assembled into the slide, and then a projection lever restricts sliding of the adapter plate in an opposite direction such that the adapter plate is not unintendedly detached from the slide.