PURPOSE: To provide a stepping device for controlling the feed of a hydroball- type detector train or detector string to a counter.

CONSTITUTION: The stepping method and device comprises two gates 22, 24 respectively driven by the solenoid-type actuators 26, 28 for checking the movement of the detector train when the hydroball-type detector train is advanced toward a gamma counter, so that the detector train in a tube 12 can be advanced by one ball at a time, at a controlled speed. The gates 22, 24 respectively comprise the paths connected with each other, one of them permits the passage of the whole detector train, but the other permits the passage of a cable of the detector trains, but does not permit that of the ball of the detector train.