The Cleveland Indians signed Tadano as a free agent in 2003, and he rose through the Cleveland farm system, making his debut in 2004. However, no Japanese team drafted him due to a gay porn video scandal he ultimately left Japan. He was a highly regarded pitcher coming out of Rikkyo University and was expected to be a high draft pick in the Nippon Professional Baseball draft in 2002. Tadano was the ace pitcher for Yachiyo Shoin High School, and led the school to the summer Koshien tournament for the first time.