World Boxing Council (WBC) strawweight champion Kazuto Ioka outpointed the World Boxing Association (WBA) minimumweight champion Akira Yaegashi by a unanimous decision on Wednesday.
An American judge scored it 115-114, while Japanese and Thai judges counted it 115-113 all in favour of Ioka.
"Yaegashi is really a strong fighter. I couldn't have beaten him without the support of my fans on my home ground," said Ioka, 23.
"Since he became my next opponent, I was under heavy pressure and I felt anxious. I felt pressure whenever I was outside the ring, but now I (have) cleared a big hurdle. But this is merely a passage in my career," he added.
The two boxers fought squarely in the first four rounds, drawing 38-38, 38-38, 38-38, with Yaegashi having his left eyelid swollen in the second round.
Ioka took a 2-1 split lead after eight rounds and drew blood from Yaegashi's nose in the 11th round before being declared the winner.
It was Ioka's third defence of the title, which he took from Oleydong Sithsamerchai of Thailand in February last year, stretching his unbeaten record to 10 wins, including six KOs.
Yaegashi saw his record reduced to 15 wins, including eight KOs, against three defeats.