The purpose of the present invention is to provide a high-concentration protein solution production method for producing a protein-rich solution by concentrating a dilute protein such as ascetic fluid, said method enabling the production of a protein-rich solution having a high protein concentration without undergoing the deterioration in concentration efficiency caused by clogging or without employing an additional treatment time such as a time for an additional concentration step. The present invention is a method for producing a high-concentration protein solution, which comprises a first step of allowing a low-concentration protein solution stored in a storage container to pass through a polysulfone-based hollow-fiber-membrane-type filter for ascetic fluid concentration use, which is a filter having ultrafiltration performance of 85 to 150 mL/min/200 mmHg and having a hydrophilic polymer attached thereto, from the storage container via a circuit and discharging a filtrate through a filtration-side outlet port of the filter and, at the same time, discharging a high-concentration protein solution through an outlet port of the filter, and a second step of collecting the high-concentration protein solution that has been discharged through the outlet port of the filter into a collection vessel, wherein the first step comprises a first substep of allowing the low-concentration protein solution to pass through the filter for ascetic fluid concentration use at a first flow rate and a second substep of allowing the low-concentration protein solution to pass through the filter for ascetic fluid concentration use at a second flow rate that is higher than the first flow rate at a time point at which a portion having a predetermined volume or more of the total volume of the low-concentration protein solution is discharged.本發明之目的在於提供一種高濃度蛋白質溶液之製造方法，其係濃縮腹水等稀薄之蛋白質溶液而獲得濃厚之蛋白質溶液的方法，且不引起由於堵塞而導致之濃縮效率下降，不追加追加濃縮步驟等之處理時間，而可獲得較高蛋白質濃度之濃厚蛋白質溶液。本發明係一種高濃度蛋白質溶液之製造方法，其包括：第1步驟，其自貯存有低濃度蛋白質溶液
