A blood purification apparatus comprising: a drain-liquid temporary chamber capable of storing drain liquid drained from a blood purifier that purifies blood of a patient; a first drain-liquid drain line (L3a) through which the drain liquid can flow into the drain-liquid temporary chamber; a second drain-liquid drain line (L3b) through which the drain liquid stored in the drain-liquid temporary chamber can be drained to an outside of the apparatus; a draining unit provided as a drain-liquid transfer pump to the second drain-liquid drain line (L3b) and that is configured to drain the drain liquid stored in the drain-liquid temporary chamber to the outside of the apparatus by transferring the drain liquid; a drain-liquid drain pump provided to the first drain-liquid drain line (L3a) for delivering dialysate from the blood purifier to the drain-liquid temporary chamber; a remaining-amount-detecting unit that is configured to detect an amount of drain liquid remaining in the drain-liquid temporary chamber; a judging unit that is configured to judge whether or not a reference remaining amount is reached by the drain liquid in the drain-liquid temporary chamber from a result of detection by the remaining-amount-detecting unit; and a control unit that is configured to control the draining unit, wherein the control unit is configured to execute a draining process in which the draining unit is controlled such that the drain liquid in the drain-liquid temporary chamber is drained to the outside of the apparatus, and the draining process is ended if it is judged by the judging unit that the reference remaining amount is reached by the drain liquid in the drain-liquid temporary chamber, and wherein the control unit is further configured to execute a monitoring process in which the amount of drain liquid in the drain-liquid temporary chamber is monitored by the remaining-amount-detecting unit, and operation is shifted from the draining process to the monitoring process if it is judged by the judging unit that the reference remaining amount is reached by the drain liquid in the drain-liquid temporary chamber, characterised in that the control unit is configured to alternately execute the draining process and the monitoring process, and, if a total amount of flow generated by the drain-liquid drain pump in a previous monitoring process is reached by a total amount of flow generated by the drain-liquid transfer pump in the draining process, the control unit is configured to shift the operation to the monitoring process. a drain-liquid temporary chamber capable of storing drain liquid drained from a blood purifier that purifies blood of a patient; a first drain-liquid drain line (L3a) through which the drain liquid can flow into the drain-liquid temporary chamber; a second drain-liquid drain line (L3b) through which the drain liquid stored in the drain-liquid temporary chamber can be drained to an outside of the apparatus; a draining unit provided as a drain-liquid transfer pump to the second drain-liquid drain line (L3b) and that is configured to drain the drain liquid stored in the drain-liquid temporary chamber to the outside of the apparatus by transferring the drain liquid; a drain-liquid drain pump provided to the first drain-liquid drain line (L3a) for delivering dialysate from the blood purifier to the drain-liquid temporary chamber; a remaining-amount-detecting unit that is configured to detect an amount of drain liquid remaining in the drain-liquid temporary chamber; a judging unit that is configured to judge whether or not a reference remaining amount is reached by the drain liquid in the drain-liquid temporary chamber from a result of detection by the remaining-amount-detecting unit; and a control unit that is configured to control the draining unit, wherein the control unit is configured to execute a draining process in which the draining unit is controlled such that the drain liquid in the drain-liquid temporary chamber is drained to the outside of the apparatus, and the draining process is ended if it is judged by the judging unit that the reference remaining amount is reached by the drain liquid in the drain-liquid temporary chamber, and wherein the control unit is further configured to execute a monitoring process in which the amount of drain liquid in the drain-liquid temporary chamber is monitored by the remaining-amount-detecting unit, and operation is shifted from the draining process to the monitoring process if it is judged by the judging unit that the reference remaining amount is reached by the drain liquid in the drain-liquid temporary chamber, characterised in that the control unit is configured to alternately execute the draining process and the monitoring process, and, if a total amount of flow generated by the drain-liquid drain pump in a previous monitoring process is reached by a total amount of flow generated by the drain-liquid transfer pump in the draining process, the control unit is configured to shift the operation to the monitoring process.