We realized cavity-enhanced absorption spectroscopy in the terahertz spectral region by combining multilayer mirrors with an attenuated total reflection technique. Using this technique, we were able to observe an absorbance of biological samples with a minimum detectable concentration as low as 8×10−10mol∕mm2. The absorbance between 75cm−1 and 115cm−1 was measured using a monolithic resonator of high-purity silicon. We demonstrate that the sensitivity of the attenuated total reflection design is significantly improved by adding multilayer mirrors for the THz region.