PURPOSE: A heat exchanger reducing power consumption by heating fluid to a desirable temperature with less fuel in a short time is provided to reduce electricity for the heating of fluid and to heat fluid in a short time by using waste heat. CONSTITUTION: A heat exchanger comprises main and auxiliary heat exchanging units(3,4) and a plurality of heaters(2). The main heat exchanging unit has a vent and stores fluid to the inner side. The auxiliary heat exchanging unit is combined with the main heat exchanging unit. The auxiliary heat exchanging unit comprises a plurality of chambers(41) accepting the fluid. The auxiliary heat exchanging unit delivers the fluid flowing from an inlet port via chambers to the main heat exchanging unit. A plurality of heaters is combined in the main heat exchanging unit and chambers.
