To prepare dense slurry not containing bubbles without contamination of work environment and without special technology.

A plastic film bag 2 storing powder plaster material 1 and water is closed by a little loosely winding an adhesive tape 14 enough to a little release the air from the upper end of the bag 2, and kneaded with hands. Subsequently, the bag 2 is put in a case body 100a of a sealed case 100, the case is evacuated, and vibration is applied to the bag. Thus, the interior of the bag 2 is also evacuated, and well mixed dense plaster slurry is formed. Subsequently, the bag 2 is taken out to collect the plaster slurry 20 at the corner part 2a. The corner part 2a is cut off by a pair of scissors, so that while the bag 2 is pressed by hands in such a manner as to operate "a squeezing bag", the plaster slurry 20 is used.