Mitoizumi was discovered by Takamiyama, a famous Hawaiian born sumo wrestler, who met the 16-year-old and his brother at a department store where Takamiyama was making a personal appearance. He was persuaded to join Takasago stable and made his professional debut in March 1978. Initially fighting under his own surname of Koizumi, he switched to the "shikona" of Mitoizumi (reference to his birthplace) in 1981. He was troubled early in his career by illness and in 1982 he seriously injured his knee and was hospitalised for four months, causing him to miss tournaments and plunge down the rankings. This was just one of many injuries he would have to battle with over the course of his long career.