Abstract: A heat exchanger has (i) a first passage in which a first fluid flows, (ii) a heat storage body that is thermally connected to the first passage and stores a warm heat or a cold heat, and (iii) a second passage that is thermally connected to both of the first passage and the heat storage body, the second passage in which a second fluid flows. The heat storage body changes to a first phase in a solid state when a temperature of the heat storage body is lower than or equal to a phase transition temperature, and changes to a second phase in a solid state when the temperature of the heat storage body exceeds the phase transition temperature. The heat storage body stores or dissipates heat depending on a phase transition between the first phase and the second phase.