As computers and factory automation machines become more popular, the stability and reliability of power sources have become an important social issue. It is difficult to completely prevent the instantaneous voltage drop and power failure of power sources. Uninterruptible power supplies (UPSs) are widely used to overcome this problem. This paper proposes a single-phase UPS with small size and improved reliability. The proposed UPS consists of a buck-boost converter, full-bridge inverter without a bulky electrolytic capacitor, and an output LC filter. The buck-boost converter regulates the output voltage to a full-wave rectification waveform. A sinusoidal output voltage can be obtained by the polarity switching of the full-bridge inverter. In this study, the experiments utilize the pilot UPS system and a small film capacitor of 40[µF], and the output power is 500[W]. The experimental results indicate that the proposed UPS supplies stable power to the load under any condition. The instantaneous interruption time of the proposed system is short, at 1[µs] with a semiconductor-type ACSW and the proposed control method. Furthermore, the experimental results confirm the time of 1[ms] when a power failure occurs.