PURPOSE: To perform operation in safety by holding the supply of dilute gas proper, by adjustig the quantity of supply of dilute gas according to the quantity of coated paint and transportation speed of a treated material or baking temperature.

CONSTITUTION: In a zone where the amount of a vaporing solvent is large, the quantity of dilute gas supplied is relatively increased according to the quantity and the concentration of solvent vaporing gas is held nearly constant in respctive zones A1WAn by previously adjusting ratio setters B1'WBn' to set the supply ratio of the dilute gas to a constant value as to respective zones A1WAn. According to the quantity of paint to be applied, the transporting speed of treated material 1 or baking temperature which are determined by the surface area, etc., of treated material 1, dilute flow arithmetic device 6 is set so as to send the command of the total supply amount to controller 7. For suction, adjustments by zones can be made by flow control valves C1WCn. Said controller 7 controls flow control valve 4 automatically to evade dangerous explosion due to a deficiency in blast as well as poor economy due to more than required dilute gas blasting.