PURPOSE: To provide an ink jet printer where ink refilling frequency is decreased when the identical sub ink tank is used, printing speed is increased when the number of pages is multiple, and a small-sized sub ink tank is installed when ink refilling frequency of the identical degree is enough, by making the best use of ink in the sub ink tank.

CONSTITUTION: The amount of ink to be used necessary to print an image on a sheet of printing paper is counted (S13), and the amount of ink is compared with the amount of ink in a sub ink tank detected by an ink sensor which detects the remaining amount of ink in the sub ink tank (S14). In the case of a small amount of ink in the sub ink tank, the sub ink tank is refilled with ink from a main tank by a pump (S17). In the case of a large amount of ink in the sub ink tank, an image is printed on a sheet of printing paper by an injection head with the sub ink tank not refilled with ink.