The present disclosure relates to an uninterruptible power supply (UPS) system and, more specifically, to a static transfer switch (STS) that can be applied to a UPS module, the static transfer switch comprising: one semiconductor rectifying element connected to either the anode terminal or the cathode terminal of a direct current power source; a bypass circuit for connecting the input terminal and the output terminal of the semiconductor rectifying element so as to bypass the semiconductor rectifying element; a breaker for opening or closing the bypass circuit; and a switch including a control unit, which controls the semiconductor rectifying element so as to conduct current when a preset conduction signal is received, controls the breaker so as to close the bypass circuit, and, when the bypass circuit is closed by the breaker, controls the semiconductor rectifying element so as to stop the conduction of current.