This paper aims to review the process of mobile phone operators agreeing to introduce mobile number portability (MNP) in Japan. A study group formed by the Ministry of Internal Affairs and Communications estimated the proportion of MNP users and analyzed the user benefits and associated costs to introduce MNP. Owing to the particular market structure and technology available, MNP was expected to yield a larger benefit in the Japanese mobile market than that seen in other countries. Based on the results of the research reviewed with respect to the market situation one year after introducing MNP, the increased competition in the mobile phone market was onfirmed by decreasing call charges and a reduction in the share of the dominant operator. (C) 2012 Elsevier Ltd. All rights reserved.