An ink cartridge consists of an ink bag storing ink and a case holding the ink bag. A valve unit is placed on an ink supply path from the ink bag to a recording head. During the printing, the amount of ink discharged from the recording head is measured at predetermined time intervals, and the valve unit is controlled to supply the recording head with the ink by an amount corresponding to the discharged amount. So the pressure inside the recording head is kept approximately constant, improving stability of ink-discharging from nozzles of the recording head. A head internal pressure detecting mechanism detects pressure inside a sub-tank as a value representative of the head internal pressure.