A method comprising: mixing a polymerizable monomer and a polymerization initiator at a mixing temperature of 1-60°C for a mixing period of 1 minute-24 hours to prepare a mixed polymerizable monomer, preparing a silane treatment liquid containing 1-40 % of silane coupling agent, and 99-60 % of organic solvent and/or water by mixing, charging the silane treatment liquid into a filler in a mixing container, mixing the filler and the silane treatment liquid at a mixing temperature of 1-60°C for a mixing period of 1 minute-24 hours to prepare a silanated filler, preserving the silanated filler in a silanated filler preserving container for 30-600 days of preserving period, kneading the mixed polymerizable monomer and the silanated filler at a kneading temperature of 5-60°C for a kneading period of 5-40 minutes after charging the silanated filler into the mixed polymerizable monomer, wherein a ratio of the silanated filler to the mixed polymerizable monomer is 0.1-9 parts by weight of the silanated filler to 1 part by weight of the mixed polymerizable monomer, defoaming the mixed polymerizable monomer and the silanated filler at 5-200 Torr for a defoaming period of 5-30 minutes to prepare a composite material, preserving 1-8 liters of the composite material in a composite material preserving container at a preserving temperature of 1-25°C for a preserving period of 10 days-1.5 years, transferring a portion of the composite material into a preserving container having 1-50 cc of volume, and preserving the transferred composite material in the preserving container having 1-50 cc of volume at a preserving temperature of 1-40°C for a preserving period of 50 days-5 years. mixing a polymerizable monomer and a polymerization initiator at a mixing temperature of 1-60°C for a mixing period of 1 minute-24 hours to prepare a mixed polymerizable monomer, preparing a silane treatment liquid containing 1-40 % of silane coupling agent, and 99-60 % of organic solvent and/or water by mixing, charging the silane treatment liquid into a filler in a mixing container, mixing the filler and the silane treatment liquid at a mixing temperature of 1-60°C for a mixing period of 1 minute-24 hours to prepare a silanated filler, preserving the silanated filler in a silanated filler preserving container for 30-600 days of preserving period, kneading the mixed polymerizable monomer and the silanated filler at a kneading temperature of 5-60°C for a kneading period of 5-40 minutes after charging the silanated filler into the mixed polymerizable monomer, wherein a ratio of the silanated filler to the mixed polymerizable monomer is 0.1-9 parts by weight of the silanated filler to 1 part by weight of the mixed polymerizable monomer, defoaming the mixed polymerizable monomer and the silanated filler at 5-200 Torr for a defoaming period of 5-30 minutes to prepare a composite material, preserving 1-8 liters of the composite material in a composite material preserving container at a preserving temperature of 1-25°C for a preserving period of 10 days-1.5 years, transferring a portion of the composite material into a preserving container having 1-50 cc of volume, and preserving the transferred composite material in the preserving container having 1-50 cc of volume at a preserving temperature of 1-40°C for a preserving period of 50 days-5 years. <EOS>