A material blending tank 1 is formed in a pressure-resistant, and tightly closable vessel, in its inner cavity, into which predetermined quantities of starting materials are charged, followed by hermetically sealing the tank; a material adjusting tank 2 is formed in a large size, into which a predetermined quantity of solvent is filled so as to be able to pump out from its bottom part by means of a pump P; the pump P functioning to pressure-feeding the solvent pumped out of the material adjusting tank 2 into the inner cavity of the material blending tank 1 through its bottom side. In the course of the pressure-feeding of the solvent, when the air within the material blending tank 1 is compressed to a high pressure level, the operation of the pump P is stopped and the solvent which has been pressure-fed into the material blending tank 1 is subjected to reverse flow through the inner cavity of the pump P into the material adjusting tank 2 by the pressure of the air compressed in the material blending tank 1. Subsequently, the process steps of: the pressure-feeding of the solvent into the material blending tank 1 due to re-starting of the pump P; and the reverse flow of the solvent into the material adjusting tank 2 due to stoppage of the pump P are repeated for a plurality of number of times to dissolve the starting materials into the solvent, thereby making it into slurry.