Abstract: Heat-accumulating microcapsule dispersion includes a microcapsule provided as a small-diameter heat-accumulating material including organic compound functioning as heat accumulating material in association with a phase change thereof accommodated within a microcapsule, heat transfer fluid medium including a number of the microcapsules dispersed stably therein, and a number of thermal boundary layer stirring particles dispersed in the heat transfer fluid medium. The stirring particles has a volume average particle diameter between 0.1.times.D and 1.9.times.D relative to a layer thickness D of a thermal boundary layer formed inside a heat exchanging passage through which the dispersion is caused to flow. The particles stir the thermal boundary layer in association with movement thereof inside the heat exchanging passage.