To use existing processing-dust collecting apparatus by mixing an ignition controlling agent dispersed from a fluidized layer into gas with dust dispersed in the gas.

A prescribed amount of an ignition controlling agent is charged into an ignition controlling agent tank 12, a dust collector is actuated, and dry air is supplied from a gas supply port 10 by pressure of approximately 0.1 MPa so that the ignition controlling agent introduced by a fluidized air flow forms a fluidized layer. Only pulverized fine powder is allowed to pass and to ascend into the fluidized layer, carried into a transfer air flow 14 reaching the dust collector 1 through a transfer pipe 6, and combined with a strongly flowing dust air flow 13. By the combination, the dust and the ignition controlling agent are mixed uniformly in the dust collector 1, and dust collected air flow 15 in which the ignition controlling agent is mixed reaches the dust collector to be deposited.