To accurately position and fix an ink cartridge on an ink cartridge mounting unit, a first engaging depression is formed in the bottom wall of the ink cartridge and a corresponding protrusion is formed in the mounting unit. Also, an ink supply port is formed in the bottom wall. When the ink cartridge is mounted on the mounting unit, the ink supply port is engaged first with an ink supply channel formed in the mounting unit and fluidly connected to the head unit. Then, the first engaging depression is brought into engagement with the corresponding protrusion in the mounting unit. A second engaging depression is further formed in the top. The engaging pawl of a locking arm engages the second engaging, depression when the locking arm locks the ink cartridge to the head unit. Further, a pair of ribs is provided on the rear side wall of the ink cartridge. The ribs oppose each other with a prescribed interval. A corresponding protrusion protrudes from the head unit and fits into the space between the interval of the ribs when the ink cartridge is mounted in the head unit.