To make a control of dust volume possible to return a dust collected from clay of equivalence into a recovered casting sand in correspondence to a sand character of a casting sand to be used in the day.

Dust generated from clay per se in a cooling rotary drum 1 is first collected and drops against two chutes 5, 6. From one chute 5, the dust equivalent to clay is additionally supplied to a recovered casting sand directly, and from the other chute 6, the dust equivalent to clay is fed or suspended by a positive/reverse operation of a positive/reverse rotation screw feeder 9 in need of necessity.