A printer includes a jet and an adjusting device. The adjusting device includes a base, a movable rod on the base, and an idle roller secured to the movable rod. The idle roller drives a piece of paper to slide. A feed roller is secured above the idle roller, and a gap between the feed roller and the idle roller has a gap height accommodating a paper thickness. A driving mechanism is in a side of the idle roller and transports the piece of paper out of the gap. During a printing process, the feed roller faces a printed side, and the idle roller faces an opposite side. The jet is secured above the printed side and between the driving mechanism and the feed roller. The idle roller moves substantially perpendicular to a printing surface, and the idle roller adjusts the gap height substantially equal to the paper thickness.