Hailing from Ōbatake, Yamaguchi, Tanaka established himself domestically with back-to-back decathlon wins at the Japanese Athletics Championships in 2004 and 2005. On his first international appearance, he won the bronze medal at the 2005 Asian Athletics Championships, finishing behind Pavel Andreev and Kim Kun-Woo. He took a third consecutive national title in 2006, a winning streak that would continue until 2009 when he was beaten by Daisuke Ikeda.