PURPOSE: To easily remove a bubble in a head, by a method wherein, when the bubble in the head is detected, the bubble is expelled by replacing ink in the head with a solvent, and the solvent is then replaced with ink.

CONSTITUTION: When a bubble detector 14 (for example, which detects a change in a motional impedance in an ink liquid chamber by observing it through an electric distortion vibrator 2) detects a bubble 7 existing at the inside of an ink-on- demand type ink jet head 10, the ink in the head 10 is pulled back to a switch valve 13 with a control circuit 15 actuated. An ink feed path of the switch valve 13 is closed off, and a solvent feed path is opened, whereby the solvent is supplied from a solvent feeder 11 to fill the head 10 with the solvent. After the switch 13 is changed over to an ink feeder (12) side and injection of the solvent in the head is then completed, the head is restored to a normal condition. The solvent is preferably the one having a surface tension of below 25 d y n e/cm, for example, ethyl alcohol and the like.