There are provided a blood purification apparatus and a priming method thereof which can reliably perform air bubble purging of a blood purification device while workability is maintained during priming. During the priming, an air purging step is performed by causing a blood pump to perform reverse rotation, and electromagnetic valves and are brought into an opened state so as to open the overflow line (L) and a priming solution supply line and so as to discharge a priming solution from an overflow line while supplying a dialysate functioning as the priming solution from the priming solution supply line.