PURPOSE: To eliminate the need to provide an ink supplying tube between a fixed portion and a mobile portion by a method in which a thick ink tank and an ink diluting device are mounted on a carriage and a diluent liquid is supplied to the ink diluting device when the carriage is stopped.

CONSTITUTION: A diluent liquid stored in an diluting tank 18 is sent to a diluent liquid supply nozzle 23 through a filter 19, a pressure pump 20, a filter 21, and a solenoid valve 22. During the period when the carriage as a mobile portion is stopped, the diluent liquid is dropwise supplied to a diluent liquid receiver 16 on the carriage, and the diluent liquid 24 in the receiver 16 is sent to an ink diluting device 15 where a thick ink sent from a thick ink tank B is diluted. The diluted ink is jetted from an ink head 9 through an ink supply system 12 and the filter 8.