This extracorporeal circulation cassette is provided with: a blood circuit (10) comprising a flexible tube and having a blood tube access portion set in a predetermined position in a device； a dialysate circuit (12) comprising a flexible tube and having a dialysate tube access portion set in a predetermined position in the device； a first panel (4) and a second panel (6) that are rigid and that integrate the blood circuit (10) and the dialysate circuit (12) while sandwiching the blood circuit (10) and the dialysate circuit (12) from both sides； and a window portion formed in a part of at least one of the first panel (4) and the second panel (6), disposed in a predetermined position in the device. The blood tube access portion and the dialysate tube access portion are positioned in the window portion.
