A control apparatus for a synchronous generator system has: a voltage instruction generator for generating voltage instructions (Vu*, Vv*, Vw*) based on an output power reference (P*) of the synchronous generator, currents (Iu, Iv) flowing through the synchronous generator and position information (&thgr;0, &ohgr;r) of magnetic poles of the synchronous generator; a zero crossing point detector for detecting a point that the voltage (Vu) of the synchronous generator passes through zero volt.; and a magnetic pole position calculator for calculating the information (&thgr;0, &ohgr;r) on the position of magnetic poles of the synchronous generator on the basis of the voltage instruction (Vu*) and the power reference (P*) when the synchronous generator is under the generation mode, and on the basis of an output signal of the zero crossing point detector when the synchronous generator is under the stand-by mode.