An apparatus for controlling a synchronous generator having a converter. A voltage detector detects a terminal voltage of a stator of the synchronous generator. A current detector detects a current flowing through the stator. A rotor position estimating part estimates a rotor position of the synchronous generator from the detected voltage and current. An active power detector detects a active power of the synchronous generator. A reactive power detector detects a reactive power of the synchronous generator or a terminal voltage detector detects an effective value of a terminal voltage of the stator. An active power controller adjusts a q-axis current command to control the active power, and a reactive power controller or terminal voltage controller adjusts a d-axis current command to control the reactive power or terminal voltage.