Disclosed is a novel means for preparing a platelet solution replaced with an artificial preservation solution, said platelet solution having excellent performances of inhibiting platelet activation, recovering platelets and removing plasma. A method of the present invention for preparing a platelet solution replaced with an artificial preservation solution comprises: a filtration step for, under the conditions represented by formula (1), using a separation membrane, said separation membrane having an average pore size of 0.1 µm or more but less than 2.0 µm in the surface to be in contact with a platelet solution, cross-flow filtering the platelet solution at an inlet wall shear rate of 25-1500 s-1 to give a concentrated platelet solution and a mixing step for mixing the concentrated platelet solution with an artificial preservation solution to give the platelet solution replaced with the artificial preservation solution. Formula (1): 1.0×1011<(A×B)/(?×µP)=2.5×1013(platelets/(hr·m2·Pa) A: concentration of platelets contained in platelet solution (platelets/mL in the range of 8.0×107-200×107 platelets/mL) B: filtration flow rate per area (mL/hr/m2) ?: inlet wall shear rate (s-1) µP: viscosity (Pa·s) of platelet solution
