Abstract We report on the Yamaguchi Interferometer (YI), a one-baseline radio interferometer consisting of two radio telescopes with diameters of 32 m and 34 m. The YI is capable of making observations at frequencies of 6.9 GHz and 8.4 GHz for left-hand and right-hand circular polarizations with a bandwidth of 512 MHz. The detection sensitivity for a continuum source is 1.6 mJy (5σ) for an integration time of 1000 s. The high sensitivity is the major advantage of the YI. We use the YI mainly for monitoring observations of sources with fast variability, such as micro-quasars.