A wake field acceleator in which a current to be formed by a driving charged particle bunch that excites a wake field is controlled so as to compensate a Joule heat loss on the wall surface of a cavity constituting the wake field accelerator, and to subject the driving charged particle bunch to a substantially uniform deceleration voltage. With the wake field accelerator, the maximum transformer ratio can be realized with a small beam length of the driving charged particle bunch, and an energy extraction efficiency of approximately 100% can be realized.