The blood processing apparatus 1 comprises a measuring instrument 8 comprising at least one chamber member 80, 81 for receiving blood fluid. The at least one chamber member 80, 81 extends along the longitudinal axis L and includes peripheral walls 804, 814 extending around the longitudinal axis L, bottom walls 803, 813 and top walls 805, 815. And these walls together define flow chambers 802, 812. At least one chamber member 80, 81 has an inlet port 800, 810 that allows blood fluid flow into the flow chamber 802, 812 and an outlet port that allows blood fluid flow out of the flow chamber 802, 812. 801 and 811. The blood processing apparatus 1 further includes a holder device 9 for holding the measuring device 8, and the holder device 9 has a base 90 having a storage opening 900 for storing the measuring device 8, and the measuring device 8 at an insertion position in the storage opening 900. And a closing member 91 movably disposed on the base 90. The ultrasonic sensor elements 92, 93 of the holder device 9 are arranged in the base 90 and generate an ultrasonic sensor signal P for measuring the hematocrit value of the blood fluid in the flow chambers 802, 812. Here, the ultrasonic sensor elements 92, 93 face the bottom walls 803, 813 of the at least one chamber member 80, 81 at the insertion position of the measuring instrument 8, and flow into the flow chambers 802, 812 through the bottom walls 803, 813. The ultrasonic signal P is transmitted. In this way, it is possible to easily insert the measuring instrument into the holder instrument, and in particular to provide a holder instrument for the measuring instrument that can reliably measure the hematocrit value of the blood flow through the measuring instrument. A blood processing apparatus is provided. [Selection] Figure 9血液処理装置１は、血液流体を受け取る少なくとも１つのチャンバー部材８０、８１を備える測定器８を備える。少なくとも１つのチャンバー部材８０、８１は、長手方向軸Ｌに沿って延在し、長手方向軸Ｌの周りに延在する周壁８０４、８１４と、底壁８０３、８１３及び頂壁８０５、８１５とを備え、これらの壁はともに流れチャンバー８０２、８１２を画定する。少なくとも１つのチャンバー部材８０、８１は、流れチャンバ
