In the present invention, a lower electrode is utilized as a photomask to form a liquid-repellent region having a generally same pattern shape as that of the lower electrode and a liquid-attracting region having a generally reversed pattern shape on an insulating film. A pair of horizontal upper and lower rails extend between and secured to the vertical posts. The phase difference between the second and third signal is determined as a second group phase delay. The frequency adjusting component 3a generates a triangle wave of a frequency based upon the output current setting signal 8, and supplies the triangle wave to an output adjusting-signal generating component 3c.