To effect printing easily onto the surface of a thick member, e.g. floor, wall or glass.

The ink jet head 2 of a self-traveling printer 1 reciprocates laterally through a carriage mechanism 3 and reciprocates back and forth through a head traveling mechanism 4 supporting the carriage mechanism 3. Desired printing can be effected onto the floor surface mounting an apparatus frame 5 or the surface of a thick medium placed between the left and right side plates 51, 52 of the apparatus frame 5 while moving the ink jet head 2 to the left and right or back and forth. Head gap can be adjusted by means of an adjusting mechanism 7 or by adjusting the supporting legs 6 of the apparatus frame 5.