The present invention relates to an automatic voltage regulator, and more specifically, to an automatic voltage regulator capable of precisely controlling the output voltage level by using a toroidal autotransformer. The present invention has precise voltage control to enable the output of the voltage level desired by the user, and precisely carries out a variety of applications of power saving and voltage booster. In particular, the present invention can boost/reduce the input voltage to provide a desired target voltage within an error range of 1 volt or less. The present invention also comprises a simple relay switching circuit and excludes semiconductor switching devices, thereby being capable of operating adaptively in different system environments without an additional modification.