A highly sensitive detector of submillimeter wavelength radiation is reported. The detector consists of a semiconductor quantum dot (QD) and a metallic single-electron transistor (SET). The SET detects change in the potential distribution induced by photon absorption within the QD. We have fabricated and studied this detector at wavelengths longer than 200 μm. High sensitivity, ∼ 10-20 WHz in terms of noise equivalent power, is found. Further optimization of the detector design is suggested.