Miyazaki was born in Kosai, Shizuoka Prefecture, Japan. He was selected for the Japanese Olympic team for the 1932 Los Angeles Olympics at the age of 15, and was thus one of the youngest members. In the semi-finals of the 100 m freestyle event, he surpassed the Olympic record previously set by American champion Johnny Weissmuller with a time of 58.2 seconds, and won the gold medal. The following day, as part of the team competing in the 4x200 m freestyle relay event, he won a second gold medal, with the team setting a new world record of 8 minutes 58.4 seconds. After his return to Japan, he entered Keio University and retired from competitive sports.