PURPOSE: To safely and effectively stop the rotation of an armature by mechanically braking a generator by means of generative braking when the rotating speed of the generator becomes a predetermined speed or lower.

CONSTITUTION: When a stop command is applied to stop the rotation of the armature of a generator 1, a breaker 3 for the generator and a field breaker 13 are first opened to interrupt a field current. Then, a disconnecting switch 2 is closed to shortcircuit an armature winding 1a. Thus, a generative braking is executed to mechanically brake the generator when the rotating speed of the armature becomes approx. 2% of a rated speed. When the generative brake malfunctions, a mechanical brake controller 51 mechanically brakes the generator by driving a mechanical brake 50 as the speed of the armature drops approx. 20% or lower of the rated speed after a counting timer 30 finishes counting of a predetermined time.