A duplex calorimeter which is connected between a first demand side and a second demand side so as to measure a calorific value of heat energy between the first and second demand sides (10, 20), comprising: a first connection pipe of which one end is connected to a pipe of the first demand side; a second connection pipe of which one end is connected to a pipe of the second demand side; first and second branch pipes (510, 520) which are branched into two directions between the other end of the first connection pipe and the other end of the second connection pipe; a first flow rate measuring part disposed at the first branch pipe so as to measure a flow rate of the heat energy passing through the first branch pipe; a second flow rate measuring part disposed at the second branch pipe so as to measure a flow rate of the heat energy passing through the second branch pipe; a temperature measuring part comprising: a first temperature measuring part disposed at the fist branch pipe to measure the temperature of the heat energy passing through the first branch pipe; a second temperature measuring part disposed at the second branch pipe to measure the temperature of the heat energy passing through the second branch pipe; a third temperature measuring part installed at the supplying pipe or the collecting pipe (410 or 420) to measure the temperature of the heat energy passing through the corresponding pipe (410 or 420); a calculating part to calculate the calorific value based on the flow rate measured by the first flow rate measuring part or the second flow rate measuring part and the temperature difference between the collecting pipe or supplying pipe (410 or 420) and the first or second branch pipe (510 or 520) measured by the temperature measuring part. a first connection pipe of which one end is connected to a pipe of the first demand side; a second connection pipe of which one end is connected to a pipe of the second demand side; first and second branch pipes (510, 520) which are branched into two directions between the other end of the first connection pipe and the other end of the second connection pipe; a first flow rate measuring part disposed at the first branch pipe so as to measure a flow rate of the heat energy passing through the first branch pipe; a second flow rate measuring part disposed at the second branch pipe so as to measure a flow rate of the heat energy passing through the second branch pipe; a temperature measuring part comprising: a first temperature measuring part disposed at the fist branch pipe to measure the temperature of the heat energy passing through the first branch pipe; a second temperature measuring part disposed at the second branch pipe to measure the temperature of the heat energy passing through the second branch pipe; a third temperature measuring part installed at the supplying pipe or the collecting pipe (410 or 420) to measure the temperature of the heat energy passing through the corresponding pipe (410 or 420); a calculating part to calculate the calorific value based on the flow rate measured by the first flow rate measuring part or the second flow rate measuring part and the temperature difference between the collecting pipe or supplying pipe (410 or 420) and the first or second branch pipe (510 or 520) measured by the temperature measuring part. a first temperature measuring part disposed at the fist branch pipe to measure the temperature of the heat energy passing through the first branch pipe; a second temperature measuring part disposed at the second branch pipe to measure the temperature of the heat energy passing through the second branch pipe; a third temperature measuring part installed at the supplying pipe or the collecting pipe (410 or 420) to measure the temperature of the heat energy passing through the corresponding pipe (410 or 420); a calculating part to calculate the calorific value based on the flow rate measured by the first flow rate measuring part or the second flow rate measuring part and the temperature difference between the collecting pipe or supplying pipe (410 or 420) and the first or second branch pipe (510 or 520) measured by the temperature measuring part.