This paper analyzes the trend and decline factors of the Japanese mobile phone industry environment and the isomorphism process that worked in the background. Furthermore, we examine the decline factors based on isomorphism, a theoretical framework for analyzing the decline factors of the Japanese mobile phone industry. The first decline factor is Galapagos ke-tai's independent mobile phone standards and technologies, the second is a vertically integration led by a mobile carrier. At the center of these decline factors were the government and NTT DoCoMo after privatization. In this process, government-led “Coercive isomorphism” and “Mimetic process” centered on NTT DoCoMo, which was later the subject of benchmarking by mobile carriers, occurred comprehensively. Japan's own mobile phone standards and unique vertical integration are factors that clearly declined the Japanese mobile phone industry, and it can be said that ‘isomorphism’ promoted this.