Disclosed is a duplex calorimeter which may measure a duplex calorific value using one device in a thermal energy network based on a duplex heat transaction. The 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 includes 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 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 measuring a temperature of the heat energy in the first branch pipe or the second branch pipe, and a calculating part calculating 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 measured by the temperature measuring part.