The nail gun contains a gun body and an adjustment device. The gun body has a base containing a top opening. The adjustment device is mounted on the base, and contains a sliding member and a positioning member. The sliding member contains two opposing elongated wing pieces extended along the two lateral sides of the base, respectively. Each wing piece has a through slot, and each slot has a number of positioning notches along a top side of the slot. The positioning member contains a positioning element and an elastic element. The positioning element is housed in the base and is exposed out of the top opening. The positioning element also contains at least a positioning bar extended out of the base into a slot to engage the positioning notches. The elastic element has its two ends connected to the positioning element and the base.