In the ink jet head described in the specification, a reservoir body contains three ink reservoirs and a vacuum reservoir and a cover plate at the front of the reservoir body provides passages to supply ink from the ink reservoirs to an ink jet array from which ink is selectively ejected in response to electrical signals. Air-permeable, ink-impermeable membranes in a lung plate adjacent to the cover plate provide one wall of the ink passages, and the opposite sides of the membranes communicate with the vacuum reservoir. The reservoir body is made of a heat-conductive material and is removably received in an insulating housing containing a cartridge heater which supplies heat to the bottom of the reservoir body.