PURPOSE: To save labor for the change-over work of a blower by equally vary ing blow-off variable quantity of the blower in the waiting condition with the blower in the blasting condition.

CONSTITUTION: A blow-off valve 36b in the blower in the waiting condition is once made to wholly open and from closing operational quantity of the blow- off valve 36b according to a prescribed signal, the blow-off variable quantity per unit signal is calculated. In the blower in the blasting condition, by using the blow-off characteristic curve C, the opening operational quantity of the blow-off valve equal to the blow-off variable quantity is obtd. By using the operational quantity as the reference, a time coming to the whole opening of the blow-off valve 36a in the blower in the blasting condition is obtd. Based on this time, the opening/closing operations of the blow-off valves 36a, 36b in both blowers are executed at the same time and the blow-off variable quantities of both blowers are equally varied. In such a way, by changing over from the blower in the blasting condition to the blower in the waiting condition, the blasting of the blower in the waiting condition is started. By this method, working load of the operator can drastically be reduced.