As a heat storage unit wherein, the volume variation of a heat storage material can be absorbed without an accompanying decrease in heat capacity, decrease in heat transfer coefficient, and oxidation and deterioration of the heat storage material due to oxygen in the air, so that the heat storage capacity can be adequately maintained and high performance can be attained, and moreover, the number of parts and the cost can be reduced, there is provided on the outside of a main member having; a fluid passage formation section which forms a fluid passage for circulating a fluid introduced from an inlet, on the inside of an outer wall section, and a heat storage material filling space formation section which forms a heat storage material filling space for filling with a heat storage material for which the volume varies corresponding to the heat storage condition, adjacent to the fluid passage, a passage switching section which selectively switches a flow-in destination of a fluid introduced from the inlet by the position of a travelling member which travels due to the volume variation of the heat storage material, to the fluid passage or to a bypass passage which bypasses the fluid passage to discharge the fluid.