To enable such points that the handleability of powder in a transport device or/and a storage device is improved, and anhydrous gypsum collected from waste gypsum is effectively utilized for an additive for a solidification material.

The powder is pressured by a compact machine, or the like, thereafter is crushed to alter the particle distribution of the powder, so as to improve the handleability of the powder in the transport device or/and storage device. The powder after crushed is classified and the whole or parts of the powder other than a predetermined particle size may be re-pressurized and crushed. The handleability of anhydrous gypsum produced by calcining waste gypsum at a temperature of ≥350°C and ≤1,000°C is improved to utilize for the additive, or the like for the solidification material. It is preferable that the mode particle size after pressurization and crushing of the anhydrous gypsum is ≥10 μm and ≤1,000 μm, and the bulk specific gravity of a shaped product after pressurized is 0.18 g/cm3. It is suitable that a compact machine is used for the pressurization of the anhydrous gypsum to raise up the linear load for shaping by the compact machine above 2.0 t/cm.