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A new type calorimeter for precise measurement of 10-W level laser power has been fabricated. This is a double configuration consisting of an absorbing unit made of a disk absorber and an isothermal type calorimeter. These components have a fast response and high accuracy. The principle of the measurement is based on the dc substitution method. The ratio of the substituted laser power to the dc power is determined by the absorbing unit. The basic structure of the system and preliminary experimental results are described. The time constant of the absorbing unit is shorter than 0.4 s. The effective efficiency of the unit is 94.0-94.1% with a standard deviation of 0.15% in the range from l to 10 W at 10.6 μm wavelength using a CO2 laser.