On 17 December 2013 he made his international refereeing debut, taking charge of the first test between Hong Kong and Belgium.
In 2014, he appeared in the 2014 Asian Five Nations, which also doubled as 2015 Rugby World Cup qualification, where he managed two matches. In 2015, Kubo return to the World Rugby Under 20 Championship, which saw him manage three games,[3] including New Zealand v Argentina and the 7th place game, Scotland v Ireland. On 10 September 2015, Kubo became the first Japanese referee to officiate an ITM Cup match, refereeing Waikato's clash against Southland. The following week he took charge of Tasman's home game against North Harbour.
In 2016, he became the first Japanese referee to referee a Super Rugby match. He was in charge of the game between the Southern Kings and the Bulls.[4] That year he also refereed three World Rugby internationals, taking charge of Canada against Russia during the June international window, before finishing the year with his first Tier 2 v Tier 2 game (Romania v Canada) having previously only refereed matches that has featured Tier 3 nations.