A method for preparing a lyophilizate of polymeric nanoparticle by a lyophilization process of a solution of polymeric nanoparticle, wherein the solution of polymeric nanoparticle comprises an amphiphilic block copolymer comprising a hydrophilic block (A) and a hydrophobic block (B) in A-B, A-B-A or B-A-B form; a polylactic acid derivative having carboxyl terminal group; and a lyophilization aid, wherein the solution of polymeric nanoparticle has a polymer concentration of 120 mg/ml or lower, wherein the lyophilization process comprises: a) a first freezing step where the solution of polymeric nanoparticle is frozen to a temperature in the range of -10 to -45 °C; b) an annealing step where the resulting product of the first freezing is heated to a temperature in the range of -25 to 0 °C; c) a second freezing step where the resulting product of the annealing is frozen to a temperature in the range of -10 to -45 °C; d) a primary drying step where the resulting product of the second freezing is dried at a temperature of lower than 0 °C under reduced pressure; and e) a secondary drying step where the resulting product of the primary drying is dried at a temperature of 0 °C or higher under reduced pressure; wherein the lyophilizate of polymeric nanoparticle is reconstituted within 5 minutes when reconstituted with an aqueous solvent under atmospheric pressure; wherein the hydrophilic block (A) is selected from the group consisting of polyethylene glycol, monomethoxypolyethylene glycol, polyvinyl pyrrolidone, polyvinyl alcohol and polyacrylamide; and the hydrophobic block (B) is selected from the group consisting of polylactide, polyglycolide, polydioxan-2-one, polycaprolactone, polylactic-co-glycolide, polylactic-co-caprolactone, polylactic-co-dioxan-2-one and polyglycolic-co-caprolactone, and derivatives thereof where the terminal carboxylic acid is substituted with fatty acid group; wherein the hydrophilic block (A) has a number average molecular weight of 500 to 20,000 Daltons; and the hydrophobic block (B) has a number average molecular weight of 500 to 20,000 Daltons; wherein the weight ratio of the hydrophilic block (A) : the hydrophobic block (B) in the amphiphilic block copolymer is 2:8 to 8:2; wherein the polylactic acid derivative having carboxyl terminal group is one having alkali metal ion bonded to the terminal carboxylic acid group or carboxyl group of one or more selected from the group consisting of D,L-polylactic acid, D-polylactic acid, polymandelic acid, copolymer of D,L-lactic acid and glycolic acid, copolymer of D,L-lactic acid and mandelic acid, copolymer of D,L-lactic acid and caprolactone, and copolymer of D,L-lactic acid and 1,4-dioxan-2-one; and wherein the polylactic acid derivative having carboxyl terminal group has a number average molecular weight of 500 to 5,000 Daltons. a) a first freezing step where the solution of polymeric nanoparticle is frozen to a temperature in the range of -10 to -45 °C; b) an annealing step where the resulting product of the first freezing is heated to a temperature in the range of -25 to 0 °C; c) a second freezing step where the resulting product of the annealing is frozen to a temperature in the range of -10 to -45 °C; d) a primary drying step where the resulting product of the second freezing is dried at a temperature of lower than 0 °C under reduced pressure; and e) a secondary drying step where the resulting product of the primary drying is dried at a temperature of 0 °C or higher under reduced pressure; <EOS>