A 280 GHz single-channel horizontal millimeter-wave interferometer system has been installed for plasma electron density measurements on the Korea Superconducting Tokamak Advanced Research (KSTAR). An electron density of plasma is measured on double-path horizontal line with triangular geometry. A cassette system contains two vacuum windows was installed on median port for these purpose. Maximum line-integrated electron density of first plasma is set to 1019 m-2 in this geometry. Since a line density of single-fringe in 280 GHz is 2 × 1018 m-2, a multi-fringe counting circuit has been adopted for a fringe-jump compensation. Measured IF signals are divided into 4 channels which has fringe counting capability of 1, 2, 4 and 8 fringes, respectively. A phase difference between IF signals is converted to DC voltage in each channel according to its fringe coverage. A fringe-jump analysis algorism has been developed for a discrimination of real fringe-jump from noise signal. An electron density of the KSTAR first plasma has been measured and analyzed using this system. Upon these results, an advanced fringe counting scheme will be proposed in this paper.