PURPOSE: To perform proper control of brake of a belt conveyor at a descendent gradient by using a speed detecting means which is constituted to eliminate a fear of a trouble occurring even in adverse environment and reduce size and a cost.

CONSTITUTION: A proximity switch 41 is arranged to a pulley 13 serving as a speed transmitting means to detect a speed actual value to detect rotation thereof. A fixed period pulse is differently generated, the number of fixed period pulses is counted between the pulse of the proximity switch 41 and a subsequent pulse and through repetition of computation wherein a machine constant is divided by the number of fixed period pulses, the speed actual value of a constant belt conveyor 2 is computed. An electric control means is controlled so that the speed actual value is caused to coincide with a speed command value, and the belt conveyor 2 operated at a descendent gradient is prevented from run away owing to the weight of a substance 3 to be carried.