The exact power output of a table-top-sized terahertz (THz)-wave source using a nonlinear optical process has not been clarified because detectors for these experiments [Si bolometer, deuterated triglycine sulfate (DTGS), etc.] are not calibrated well. On the other hand, powermeters for the mid-infrared (mid-IR) region are well established and calibrated. We constructed a high-power dual-wavelength optical parametric oscillator with two KTP crystals as a light source for difference frequency generation. The obtained powers of dual waves were 21 mJ at ~1300 nm, ten times higher than that of the previous measurement. The device provides high-power THz-wave generation with ~100 times greater output power than that reported in previous works. A well-calibrated mid-IR powermeter at ~27 THz detected the generated THz wave; its measured energy was 2.4 microJ. Although the powermeter had no sensitivity in the lower-frequency range (below 20 THz), the pulse energy at such a low-frequency region was estimated in reference to the output spectrum obtained using a DTGS detector: the energy would be from about the submicrojoule level to a few microjoules in the THz-wave region.