He led the side up to the 2003 Rugby World Cup, where Japan put up some credible performances against Scotland and France but ultimately finished the tournament winless. He stepped down after the tournament to spend more time with his family. He finished his time as Japan coach with a record of 9 wins in 24 matches.
After leaving his post with Japan, he became coach of the Coca Cola West Red Sparks in 2004. He led them to promotion to the Top League in 2006 where they stayed until they were relegated in 2012. This was Mukai's last season in charge as he handed over the reins to new coach Satoshi Yamaguchi and moved into a directors role at the club.
During his time as a coach, Mukai was renowned for employing an attacking game plan.