To obtain dental investment compd. which is low in expansion at the time of curing, is high in expansion at the time of heating and does not crack even if heating up is executed at an ordinary rate by adding tridymite and magnesium silicate into the dental investment compd. prepd. by compounding a phosphate binder and alumina cement with at least either of quartz and cristobalite.

The tridymite is gentle in the rising curve of a coefft. of thermal expansion in a region below 200°C and, therefore, if the tridymite is used in combination with the quartz and the cristobalite, the rapid thermal expansion at the time of heating a casting mold is suppressed and the occurrence of the crack is prevented. The coefft. of thermal expansion of the magnesium silicate increases linearly with the temp. and, therefore, the rapid thermal expansion at the time of heating the casting mold is suppressed and the occurrence of the crack is prevented. The compounding ratios of the tridymite and the magnesium silicate are respective preferably specified to 5 to 20 wt.% and 2 to 20 wt.% of the total amt. of the dental investment compd.