Methods are provided for transitioning between SIP and MIP. The coupling means comprise slide means 33 provided with a slide plate 24 which slides with respect to a bracket hardware 24 embedded in the unit main body, and holding means 28 provided at a front face of the slide plate 24 and for holding the front pane. When a portion intermediate between the maximum amplitude level and the dark-part amplitude level is defined as an intermediate level, the intermediate level is lower than the bright-area amplitude level. Each operation of the directional control valve reverses travel direction of the barrier within the cylinder bore and injects another fixed quantity of fluid into the well.