A thermal printer has a first thermal head, a second thermal head, and a feeding mechanism. The feeding mechanism feeds one of thermal papers which include a double-sided thermal paper having thermosensitive layers formed on both sides thereof and a single-sided thermal paper having a thermosensitive layer formed on one side thereof. The first thermal head is so provided as to be brought into contact with a first side of the thermal paper fed by the feeding mechanism. The second thermal head is so provided as to be brought into contact with a second side of the thermal paper fed by the feeding mechanism. The thermal printer determines whether a mark has been printed at least one of the first and second sides of the thermal paper and controls print operation based on a determination result.