A medical fluid passage switching apparatus comprising: at least, a rotatable shaft F; a cam C which is rotated in relation to the rotating movement of the shaft F to close and open divided tubes for determining a passage of a fluid; and a housing L having tube accepting holes I and so arranged to enable the holding of the cam C, is provided. More particularly, provided are such a medical fluid passage switching apparatus in a continuous ambulatory peritoneal dialysis (CAPD) system and a tube which is decreased in the diameter and/or thinned in the wall at a region where the cam or a combination of the cam and the clamp is engaged for closing and opening.