A heat generator includes a cubic heat generating member for outputing heat flow. The heat generating member includes a heat flow output face and five heat flow insulation faces. Five thermoelectric coolers are attached on the five heat flow insulation faces respectively. A heat flow compensating circuit is electrically connected between each heat flow insulation face and a corresponding thermoelectric cooler. The circuit is capable of controlling heat generated by the thermoelectric cooler to cause the temperature of the heating face of the thermoelectric cooler to be equal to the temperature of the heat flow insulation face which results in the heat energy of the heat flow outputing from the heat flow output face of the heat generating member substantially equal to the heat energy of the heat generated by the heat generating member.