The present invention relates to a hematocrit sensor and a method for measuring hematocrit values using such a device, which comprises a hematocrit sensor in a blood circuit that measures hematocrit value, a blood purifier that purifies a patient's blood while extracorporeally circulating the patient's blood, a housing having a slot connected to a portion of the blood circuit, a slit or plurality of pores built in the slot of the housing, and a light emission means and a light reception means positioned in the housing such that both means face the blood circuit through the slit or the plurality of pores.