When fly ash is added to a cement kiln to materialize it after being desalinated by washing, scale is prevented from growing in a dissolution reaction apparatus depositing calcium-containing compounds dissolved in slurry, and the fly ash is effectively used as a cement material. A dissolution reaction apparatus according to the invention comprises: a dissolution tub 3 for dissolving powder material, a wet dust collector 5 for collecting powder material and mist while reacting slurry S in the dissolution tub 3 with gas G and returning the collected powder material and mist to the dissolution tub 3. A second wet dust collector 7 can be provided for collecting powder material and the like accompanying to the gas G discharged from the wet dust collector 5, and both the wet dust collectors 5, 7 can be vertical type and mounted independently with each other on the dissolution tub 3. The slurry S and the gas G can react with each other with parallel flow by feeding the slurry S and the gas G to an upper portion of the first wet dust collector 5, or the powder material and the like accompanying to the gas G may be collected with counter flow by feeding the gas G discharged from the dissolution tub 3 to a lower portion of the second wet dust collector 7.