Ink cartridge and ink jet printer

An ink cartridge mountable, in a mounting direction crossing with a direction of gravity, to a cartridge mounting portion provided with an ink receiving tube and a plurality of electrical connecting portions, said ink cartridge comprising:

• a casing including an ink accommodating chamber configured to accommodate ink to be supplied to said ink receiving tube, said casing having a first side surface in a position at a leading end with respect to the mounting direction, a second side surface in a position at a trailing end with respect to the mounting direction, a third side surface connecting said first side surface and said second side surface with each other, a fourth side surface opposite from said third side surface across said ink accommodating chamber and connecting said first side surface and said second side surface with each other, an upper surface and a bottom surface;
• a tube inserting portion into which the ink receiving tube is insertable and which is provided at said first side surface in a position closer to said bottom surface than to said upper surface;
• a substrate provided at a position above said tube inserting portion;
• a plurality of electrical contacts electrically connectable with the plurality of electrical connecting portions, said electrical contacts being arranged on said substrate in a direction perpendicular to said third side surface and to said fourth side surface;
• a positioning portion provided in a position above said tube inserting portion and below said electrical contacts and engageable with a positioning member provided in the cartridge mounting portion to limit movement of said casing in a direction perpendicular to the mounting direction;
• and
• a through-opening at a position between said tube inserting portion and said positioning portion of said first side surface, said through-opening being in fluid communication with said ink accommodating chamber to supply the air into said ink accommodating chamber.