Tobacco shreds are drawn from a magazine by a pneumatic conveyor pipe which delivers the shreds into the lower chamber of an air separator whose upper chamber is separated from the lower chamber by an air-permeable partition and is connected with the inlet of a suction generating device serving to draw an air stream through the pneumatic conveyor pipe. The velocity of the air stream in the pipe is maintained at a constant value by a flow restrictor which is installed in the inlet of the suction generating device and is adjustable by hand or by a servomotor receiving signals from a device which ascertains the velocity of air in the pipe by comparing the velocity of air in the second chamber with the velocity of air in the inlet of the suction generating device. The air separator has an outlet for periodical admission of batches of tobacco shreds from the first chamber into a duct which delivers the shreds into the distributor of a cigarette rod making machine.