In 2004, Mizutani played his first game between countries. At the Youth-World Championships, he won the gold medal in the doubles but lost in the singles in the 16th round. At the Youth-World Junior Circuit in Poland, Mizutani only made it to the quarterfinals, where he lost to Maharu Yoshimura. In 2005, Mizutani played in his first World Championships for adults, but did not win a single medal. He reached Round 32 in the singles. Even though this was not a win, it did increase his world ranking from 800 to 720 (the best players have the lowest numbers). That year he could have won the World Junior Circuit. At the Youth-WTTC in 2005, he won the silver medal, beating Chen Qi. His team won gold and bronze in the doubles. At his first Pro Tour tournaments, he reached Round 32. At the Asian Championships, he played well. Even though he did not win any medals, his rank went from 600 to 520. In 2006, Mizutani played at the WTTTC 2006 in Bremen, where he reached 14th place with the Japanese team. At some Pro Tour events that year, he reached the quarterfinals.