This tablet printing apparatus first irradiates ultraviolet rays or infrared rays onto the surface of a tablet (9) that is being conveyed. The ultraviolet rays or the infrared rays are irradiated onto at least an outline part of a target area (A) where a print image is to be formed. When ultraviolet rays are irradiated, the surface of the outline part is roughened to reduce an ink contact angle. When infrared rays are irradiated, the outline part is etched to form a recessed shape. Ink (I) is then ejected inside the outline part. Thus, the ejected ink (I) can be prevented from spreading outside the target area (A). As a result, the outline of the print image can be made clearly visible.