Provided is a device which can form favorable perforated images corresponding to the resolution of the thermal head, reproduce faithful printed images for all kinds of original picture images, avoid ink transfer, and adapt itself to different environmental conditions, and is suitable for use with different thermal recording materials such as thermal recording paper, OHP TP sheets, and thermal stencil master plates. In the device of the present invention, a thermal head 4 consisting of a plurality of heat emitting elements 5 arranged in a single row in the primary scanning direction is directly contacted to the recording surface of a thermal recording material such as thermal recording paper, and the thermal recording material 1 is moved relative to the thermal head 4 in the secondary scanning direction which is perpendicular to the direction of the row of the heat emitting elements so that picture images may be formed with a dot matrix by selectively heating the thermal heat emitting elements, the ratios of the length of each heat emitting element 5 in the primary and secondary scanning directions to the pitches of the primary and secondary scanning are set 30 to 70% and 60 to 95%, respectively.