Conventionally, it has been difficult to effectively and promptly stir and mix fluids together by use of a microfluidic device having a simple flow path structure. Additionally, there has been no means for keeping a particulate sample floating in a fluid in a flow path for a long time without precipitating the particulate sample. Additionally, there has been no method for measuring the true size of a flowing and floating particulate sample by use of a microscope. The present invention solves these problems by using a microfluidic device in which an electrode pair having a wide electrode-to-electrode gap is formed in a flow path or in a chamber, and by applying an AC voltage to this electrode pair, and by generating an eddy by which a fluid is swirled in a torus manner. The accurate size of the particulate sample that crosses the in-focus plane can be measured especially by setting an in-focus plane (53) of an objective lens (52) of a microscope at a position through which a swirling flow (41) vertically passes.