PURPOSE: To permit the treating of ash without dispersing the powder dust of dry ash into air by a method wherein liquid is mixed into dry ash to obtain liquid ash on the half way of a suction hose, sucking the dry ash.

CONSTITUTION: A worker W enters a flue P through the outlet/inlet port D of the flue P and sucks dry ash DA, accumulated in the flue P, by a suction hose 10. The sucked dry ash DA flows into a mixing device 30 through the hose 100. Water is mixed into the dry ash DA in the mixing device 30 whereby the dry ash becomes sludge type. The sludge of ash is reserved in the storage tanks of vacuum cars 20a, 20b through the suction hose 10. Subsequently, the sludge of ash in the storage tanks of the vacuum cars 20a, 20b is received into a reserving tank 50 through a pressure hose 52. Supernatant liquid in the storage tank 50 is discharged through a supernatant liquid tank 54 and a hose 58. Sludge except the supernatant liquid is neutralized in a neutralizing tank 60 by alkaline substance and, thereafter, is dehydrated by a dehydrating device 70 to make cake type ash.