A method of priming a leukocyte remover 12 in which a filter 5 in a substantially dry state is housed in a container 1 or 2 having a first port 3 and a second port 4 for liquid provided near the peripheral edge thereof and a space in the container is partitioned into a first port side space 7 and a second port side space 8 by the filter 5, the priming method including: arranging the leukocyte remover 12 such that the second port 4 is located in a lowest position of the leukocyte remover 12 and the first port 3 is located in a highest position of the leukocyte remover 12 leading in a physiological solution from the second port 4 and setting average liquid surface rising speed of the physiological solution in the second port side space 8 to be equal to or higher than water absorption speed of the filter 5.
