In the ink jet head unit of the present invention, an ink jet head for emitting ink in a formof droplets from nozzles formed in a nozzle face and a driving circuit for driving the ink jet head are mounted on the same surface side of the base board. A groove section is formed in the surface, extending from the nozzle face of the ink jet head to the driving circuit. In the ink jet head, a temperature measuring section is positioned in the groove section. Furthermore, there is provided a sealing member in the groove section, to prevent the flow of ink emitted from the nozzles, into the driving circuit. The sealing member is partly formed of an elastic body. Therefore, since the elastic body can restrain a change in the volume of the sealing member likely to be caused by a temperature change, it is possible to prevent the temperature change from impairing the ink jet head, consequently realizing ink projection stability.