PROBLEM TO BE SOLVED: To provide a method for preparing cleaned platelets by which a membrane of a hollow fiber membrane module is less clogged with platelets, and a protein removal rate and a platelet recovery rate can be increased.SOLUTION: A method for preparing cleaned platelets includes: (a) a diluting process for diluting a platelet suspension (b) a filtration process for filtering by a total amount filtration method by introducing the diluted platelet suspension diluted in the diluting process (a) in an inflow side space of a hollow fiber membrane module and for removing protein molecules in the platelet suspension by making the protein molecules pass into a filtration side space (c) a cleaning process for introducing a cleaning liquid into the inflow side space to remove the protein molecules remaining in the inflow side space and (d) a recovery process for introducing a preservation liquid in the inflow side space to recover platelets existing in the inflow side space. The diluting process (a) includes the dilution to obtain the platelet suspension with a protein concentration of 55 mg/mL or less. In addition, a value of a maximum flow rate/water permeability in the filtration process is 0.15 kPa or less.
