A blood purification apparatus comprising: an arterial blood circuit provided with an arterial puncture needle (a) at a distal end, the arterial puncture needle (a) being stickable into a patient; a venous blood circuit provided with a venous puncture needle (b) at a distal end, the venous puncture (b) needle being stickable into the patient; and a blood purification device connected to a proximal end of the arterial blood circuit and to a proximal end of the venous blood circuit, the device configured to purify blood of the patient that extracorporeally circulates through the arterial blood circuit and the venous blood circuit, characterized in that the blood purification apparatus includes an arterial electrode configured to apply a voltage through it to the blood flowing in the arterial blood circuit; a venous electrode configured to apply a voltage through it to the blood flowing in the venous blood circuit; a power source configured to cause a current to flow in the blood through the arterial puncture needle (a) and the venous puncture needle (b) that are stuck in the patient, by applying the voltage between the arterial electrode and the venous electrode; a body-surface electrode configured to be closely attached to a body surface of the patient and to detect an electrical signal from a body of the patient; a detection device configured to acquire a predetermined vital parameter by detecting a change in an impedance in the body of the patient in accordance with the electrical signal detected by the body-surface electrode; a monitoring device configured to monitor in real time the current flowing through the arterial electrode or the venous electrode, and the change in the impedance in the body of the patient that is detected by the detection device; and an identifying device configured to identify a dislodged state where the arterial puncture needle (a) or the venous puncture needle (b) is dislodged from the patient, in accordance with the change in the impedance that is an object of monitoring by the monitoring device. an arterial blood circuit provided with an arterial puncture needle (a) at a distal end, the arterial puncture needle (a) being stickable into a patient; a venous blood circuit provided with a venous puncture needle (b) at a distal end, the venous puncture (b) needle being stickable into the patient; and a blood purification device connected to a proximal end of the arterial blood circuit and to a proximal end of the venous blood circuit, the device configured to purify blood of the patient that extracorporeally circulates through the arterial blood circuit and the venous blood circuit, characterized in that the blood purification apparatus includes an arterial electrode configured to apply a voltage through it to the blood flowing in the arterial blood circuit; a venous electrode configured to apply a voltage through it to the blood flowing in the venous blood circuit; a power source configured to cause a current to flow in the blood through the arterial puncture needle (a) and the venous puncture needle (b) that are stuck in the patient, by applying the voltage between the arterial electrode and the venous electrode; a body-surface electrode configured to be closely attached to a body surface of the patient and to detect an electrical signal from a body of the patient; a detection device configured to acquire a predetermined vital parameter by detecting a change in an impedance in the body of the patient in accordance with the electrical signal detected by the body-surface electrode; a monitoring device configured to monitor in real time the current flowing through the arterial electrode or the venous electrode, and the change in the impedance in the body of the patient that is detected by the detection device; and an identifying device configured to identify a dislodged state where the arterial puncture needle (a) or the venous puncture needle (b) is dislodged from the patient, in accordance with the change in the impedance that is an object of monitoring by the monitoring device. an arterial electrode configured to apply a voltage through it to the blood flowing in the arterial blood circuit; a venous electrode configured to apply a voltage through it to the blood flowing in the venous blood circuit; a power source configured to cause a current to flow in the blood through the arterial puncture needle (a) and the venous puncture needle (b) that are stuck in the patient, by applying the voltage between the arterial electrode and the venous electrode; a body-surface electrode configured to be closely attached to a body surface of the patient and to detect an electrical signal from a body of the patient; a detection device configured to acquire a predetermined vital parameter by detecting a change in an impedance in the body of the patient in accordance with the electrical signal detected by the body-surface electrode; a monitoring device configured to monitor in real time the current flowing through the arterial electrode or the venous electrode, and the change in the impedance in the body of the patient that is detected by the detection device; and an identifying device configured to identify a dislodged state where the arterial puncture needle (a) or the venous puncture needle (b) is dislodged from the patient, in accordance with the change in the impedance that is an object of monitoring by the monitoring device.