Provided is a priming method including a step of filling a priming liquid in a hollow-fiber membrane module in which hollow-fiber membranes are packed in a vessel having an inlet port, an outlet port, and a filtrate discharge port at a linear velocity of 20 cm/min or more and 550 cm/min or less through the inlet port or the outlet port in an amount of 15% or more relative to a volume of the hollow-fiber membrane module. According to the present invention, the effective filtration area at the time of cell suspension treatment is increased, and the recovery rate of cells and the filtration rate can be improved. Further, since the cell treatment can be completed while maintaining the closed environment, the obtained cells can be provided for therapeutic applications.