A material set for forming a three-dimensional object, comprising: a powder for forming a three-dimensional object, where the powder comprises an organic material and a base material; and a liquid for forming a three-dimensional object, where the liquid comprises a solvent, a cross-linking agent cross-linkable with the organic material, and at least one compound selected from the group consisting of sodium gluconate, 2-amino-2-methyl-1,3-propanediol (AMPD), 2-amino-2-ethyl-1,3-propanediol (AEPD), salts of dihydroxy ethyl glycine (DHEG) and a compound that comprises a phosphono group, wherein the base material is a base material non-reactive with the liquid for forming a three-dimensional object, wherein the cross-linking agent comprises a zirconium compound, and wherein a rate of viscosity change of the liquid for forming a three-dimensional object between before and after the liquid is left to stand still at 50°C for three days is lower than 20%, wherein the rate of viscosity change of the liquid for forming a three-dimensional object is measured by leaving the liquid for forming a three-dimensional object to stand still in a thermostat bath of 50°C for three days while being put in a wide-mouthed bottle made of polypropylene (50 mL); taking it out from the thermostat bath; leaving to stand still until the liquid for forming a three-dimensional object reaches room temperature (25°C); measuring the viscosity of the liquid for forming a three-dimensional object; calculating the rate of viscosity change according to the formula below where viscosity before storage refers to the viscosity of the liquid for forming a three-dimensional object before being put in the thermostat bath and viscosity after storage refers to the viscosity of the liquid for forming a three-dimensional object after being taken out from the thermostat bath; and wherein the viscosity before storage and the viscosity after storage are measured with an R viscometer at 25°C, Rate of viscosity change % = viscosity after storage − viscosity before storage / viscosity before storage × 100. a powder for forming a three-dimensional object, where the powder comprises an organic material and a base material; and a liquid for forming a three-dimensional object, where the liquid comprises a solvent, a cross-linking agent cross-linkable with the organic material, and at least one compound selected from the group consisting of sodium gluconate, 2-amino-2-methyl-1,3-propanediol (AMPD), 2-amino-2-ethyl-1,3-propanediol (AEPD), salts of dihydroxy ethyl glycine (DHEG) and a compound that comprises a phosphono group, wherein the base material is a base material non-reactive with the liquid for forming a three-dimensional object, wherein the cross-linking agent comprises a zirconium compound, and wherein a rate of viscosity change of the liquid for forming a three-dimensional object between before and after the liquid is left to stand still at 50°C for three days is lower than 20%, wherein the rate of viscosity change of the liquid for forming a three-dimensional object is measured by leaving the liquid for forming a three-dimensional object to stand still in a thermostat bath of 50°C for three days while being put in a wide-mouthed bottle made of polypropylene (50 mL); taking it out from the thermostat bath; leaving to stand still until the liquid for forming a three-dimensional object reaches room temperature (25°C); measuring the viscosity of the liquid for forming a three-dimensional object; calculating the rate of viscosity change according to the formula below where viscosity before storage refers to the viscosity of the liquid for forming a three-dimensional object before being put in the thermostat bath and viscosity after storage refers to the viscosity of the liquid for forming a three-dimensional object after being taken out from the thermostat bath; and wherein the viscosity before storage and the viscosity after storage are measured with an R viscometer at 25°C, Rate of viscosity change % = viscosity after storage − viscosity before storage / viscosity before storage × 100.