The present authors stndied on special dolomite magnesia plaster from semi-calcined dolomite and bittern solution from salt making plant. The main points of the study are as following. (1) Raw dolomite sample obtained from Kuzuu, Tochigi Prefecture was calcind at about 700°C far 1 hour to obtain semi-calcinned dolomite (CaCO₃·MgCO₃→ CaCO₃+ MgO), (2) Bittern abtained from salt making was diluted to the solution of various concentration Bé 20, 22 and 24°, (3) 10-30 mm grains of this semi-calcined dololmite was slaked with bittern solution 15-30% for 10-15 hours aging to hardened mass (CaCO₃+mMgO. yMgCl₂-zH₂O or CaCO₃+mMg (OH) ₂. MgCl₂. nH₂O), (4) This hardened mass was again calcined about at 300-350°C for 1 hour and then ground to the sample of special dolomite magnesia plaster (CaCO₃ + mMaO+MgCl₂), & (5) This sample was tested on its chemical compositions and various physical properties to prove to be the special superior plaster betweeh common dolomite plaster and magnesia cement. This plaster has the special property of setting and and hardening by kneeding only with water and having superior hardening strength and adhesive power.