A microchannel device includes: a microchannel that forms two or more different fluids as a multiphase flow, and a section shape of the multiphase flow having in at least one portion a first shape in which plural polygons are two-dimensionally arranged, and adjacent to one another, a second shape in which a polygon is inscribed in one of a circle and an ellipse, or circumscribes one of a circle and an ellipse, or a third shape in which at least plural circles and polygons are two-dimensionally arranged, and adjacent to one another.