Provided is a method of mixing first and second liquids having mutual solubility inside a mixing flow channel formed by a micro flow channel. This method includes: causing the first and second liquids to be joined to each other inside the mixing flow channel; and forming a slug flow, in which mixing subject cells (60) formed by the joined liquid and insoluble fluid cells (63) formed by an insoluble fluid are alternately arranged, inside a flow channel at the downstream side of an insoluble fluid supply position in a manner such that the insoluble fluid having insolubility with respect to both mixing subject liquids is supplied to the joined liquid flowing through the flow channel in a direction intersecting the flow channel so that the joined liquid is divided with a gap therebetween, thereby mixing the first mixing subject liquid and the second mixing subject liquid contained in each mixing subject cell inside the downstream flow channel.