Seiya joined Indonesia Petroleum Ltd (INPEX) in 1977 following graduation from Waseda University's Faculties of Law and Foreign Studies. He has remained with the company since this time, undertaking roles in various divisions including as the Director/General Manager Corporate Planning & Management, Director/Assistant COO Corporate Strategy & Planning Unit, and Director/Deputy COO Oceania & Americas Business Division.

Following the October 2008 restructure of INPEX CORPORATION, Seiya assumed leadership of the newly established Ichthys Project Division at INPEX Tokyo head office. He retains this position during his tenure in Australia.