To obtain a composition which can completely compensate curing shrinkage or can realize further expansibility, if necessary, by mixing an ordinary-temperature- curing synthetic resin with an at least specified amount of a heat-expandable substance and a filler powder.

This composition is obtained by mixing 100 pts.wt. ordinary- temperature-curing synthetic resin with at least 0.5 pt.wt. heat-expandable substance being a heat-expandable hollow body having a heat expansion initiation temperature of 60-160°C and a particle diameter of 100 μm or below and a filler. This composition is poured into an elastic model-molding die which can accept heat expansion at ordinary temperature. During the period from the time when the generation of curing heat of the resin reaches the peak to the time when the resin modeling material being cured still keeps plasticity under conditions of the heat expansion initiation temperature of the heat-expandable substance, the die is wholly heated to the heat expansion initiation temperature of the heat expandable substance to make a synthetic resin model. The expansion value of the cured model can be suitably adjusted by the timing of heating, the heating temperature and the heating time as well as by the amount of the heat-expandable substances added.