A pasty dental alginate impression material composition having features that fluidity of a mixture is changeable by changing mixing ratio of both pastes, sufficient working time can be kept, and liquid separation in abase material paste does not occur during storage, comprises the base material paste composed mainly of 1-20 wt. % alginate and water, and a setting material paste composed mainly of 10-60 wt. % calcium sulfate and a liquid component unreactive with the calcium sulfate, wherein the base material paste does not contain a filler substantially, but contains 0.01-5 wt. % carbonate of sodium or potassium as a retarder and 0.01-15 wt. % one or more polysaccharides such as carrageenan, pullulan, curdlan, xanthan gum, pectin, konjak glucomannan, and locust bean gum, and the setting material paste contain 0.01-10 wt. % phosphate of sodium or potassium as a retarder, and 0.5-30 wt. % polybutene.