An additive is dispersed in a matrix having a larger thermal expansion coefficient, which forms a film that prevents the permeation of the ink at the room temperature while allowing the permeation of the ink at high temperature. An ink roller is formed by covering a roller-like sponge body containing ink (an ink holder) with the film. Along the outer circumference of the ink roller, a thermal head that heats the surface of the ink roller according to a print pattern and a platen roller that sandwiches recording sheet between itself and the ink roller are disposed. Then, by rotating the ink roller and heating the thermal head, the ink permeates through the heated part of the film and is transferred to the recording sheet to form an ink image on the recording sheet.