PURPOSE: To enable an easy, rapid, and correct measurement of a proportion of T cell and B cell, by a method wherein a lymphocyte suspension and B cell suspension are obtained from a leukocyte suspension, and the number of cells in each suspension are measured.

CONSTITUTION: The leukocyte suspension in a test liquid storage receptacle 5 is fed in each equal amount into a lymphocyte separator 6 and B cell separator 7, and is caused to penetrate into a separating agent with which the separators are filled. Instantly, or after the suspension is held in a stationary manner for a specific period of time, a cleansing liquid in a receptacle is supplied into the separators 6 and 7, where the separating agent is washed, and unabsorbed lymphocytes and the B cells are fed out of the separators 6 and 7 togetherwith the cleansing liquid to be stored in receptacles 8 and 9, respectively. The number of the cells in the lymphocyte suspension and the B cell suspension, which are stored therein, are measured by means of a detector 2 and are inputted to an arithmetic unit 3 where the number of T cells and the proportion of the T cell and the B cell in the leukocyte and lymphocyte are computed.