The Japanese fighter delighted his home crowd in Sendai after his opponent was counted out, 36 seconds into the seventh, after a big left-hand.
It was Yamanaka's second successful defence of the title and he improves his record to 17-0-2.
Compatriot Toshiyuki Igarashi made a successful first defence of his WBC flyweight title as he outpointed Argentina's Nestor Narvaes.
Igarashi edged the contest 114-112, 114-112, 113-113.
- Sports & Recreation
- Toshiyuki Igarashi