PURPOSE: To always supply an optimum ignition energy irrespective of a rotating speed of an AC generator by advancing an ignition timing in response to the rotating speed of a rotor and automatically advancing an exciter output timing.

CONSTITUTION: An AC generator 1 is composed of a rotor 2, a stator, etc., and the magnets 10∼15 of the rotor 2 are formed to be movable. The rotor 2 is formed of a cylinder rotated by engaging a crankshaft 19 of an engine within the bore of a shaft 3, and clamping pins 6∼9 are arranged on an annular groove 2A between the outer peripheral wall 4 and the inner peripheral wall 5. The sliding surfaces 10 of magnets 10∼15 are formed at the ends of the pins 6∼9. Thus, the rotor 2 is rotated in a direction of an arrow, and when the rotating speed is accelerated, an ignition timing signal meeting to the rotating speed is oscillated from a pulser coil 18. Simultaneously, the magnets 12∼15 of the rotor 2 move by a centrifugal force in a centrifugal direction (phase advancing direction). As a result, the exciter timing is advanced in the amount that the ignition timing is advanced so that the maximum charging voltage time always coincides with the timing.