Before joining Milliman, Tanaka spent six years working with Mitsui Marine & Fire Insurance (which later became Mitsui Sumitomo Insurance Company Ltd.). Focusing on accident insurance, he was engaged in profit management, product development, and relevant system development in the Actuarial Section and Injury and Disease Insurance Section of the Non-Marine Underwriting Department. While working for Mitsui Marine's life insurance subsidiary, Mitsui Mirai Life Insurance Company (later renamed Mitsui Sumitomo Kirameki Life Insurance Co. Ltd. after Mitsui Marine and Sumitomo Marine merged in 2001). Tanaka gained five years experience in areas including product development, profit management, and risk management.  

Tanaka was also a member of Loss Model Study Subcommittee of the IAJ's Casualty Committee for about three years.