He became president of Sega in 1998, where he laid an ambitious plan using the Dreamcast to restore Sega's lost market share and prestige which took a severe downfall following the flawed launch of the Sega Saturn as well as the Sega 32X.
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During this time, it was said that General Motors actively recruited Irimajiri for a senior executive position at the company, in light of his success at Honda.
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He earned an aeronautical engineering degree from the University of Tokyo and worked for Honda Motor Co., Ltd. for 20 years, overseeing the introduction of its CBX motorcycle, design of their F1 racing engines for the RA273 as well as being the president of Honda of America Inc.
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After his recovery, his friend Hayao Nakayama, then president of Sega Corporation, asked him to become vice president of the company.