A nailing device of the present invention mainly includes a main body, a striker, an actuator, a resilient member, a nail slot, a magazine, a guider, an adjuster and a controlling means. The magazine has a main rail and a sub rail parallel to the main rail, and the main rail is substantially as tall as the sub rail. The adjuster has a protrusive portion having a guiding surface facing the magazine. The guiding surface is vertically formed with two parallel longitudinal grooves and has a bottom end. The longitudinal grooves extend toward the bottom end. The controlling means is for selectively pushing the adjuster to flush the guiding surface with a step surface of the guider.