PURPOSE: To prevent generation of vertical stripes by allowing a thermal head having thermal elements to come in contact with a recording medium and increasing the friction resistance of the printing surface of the recording medium compared to the friction resistance of the element surface of a thermal head when data is printed on the recording medium using heat of the thermal elements.

CONSTITUTION: When data is printed by allowing film for OHP to come in contact with a thermal head, moving relatively to the other, the surface friction resistance of the film 1 is increased compared to the surface friction resistance of a thermal element on the thermal head 2. Therefore, if foreign matter is present between the film and the thermal head, the foreign matter 3 sticks to the film instead of the thermal element of the thermal head and it moves therewith. Thus no adverse effects will be caused on the subsequent recording process so that no vertical stripes will generate. The recording media are thermally coloring-type film for OHP and frost-type film for OHP.