PURPOSE: To minimize variations in the drip speed of an ink when the volume of an ink drip is changed by overlapping the first voltage pulse with the second voltage pulse with the pulse width less than the first voltage pulse matching the rising thereof to make an electrical signal to be applied to an electromechanical means.

CONSTITUTION: An electrical signal to be applied to a piezoelectric element as electromechanical conversion means has a waveform in which the first square signal 1 varied in the pulse width corresponding to the volume of ink drops being jetted is overlapped with the second square signal 2. The second square signal 2 allows the heightening of the internal pressure of a pressure chamber in the initial stage of jetting to have the ink jetting speed vel attain the ink drip flying speed. After the ink drip flying speed is reached in a short time, the second square signal 2 is cut off and the ink jetting speed vel is kept almost constant solely depending on the first square signal 1.