A customer journey map (CJM) is a widely used tool to represent user experience with a service. Although numerous companies have used this tool to improve existing services or to develop new services, the maps are neither consistent nor mutually compatible because no clear design process for a CJM has been presented. This study aimed to develop a design process and rule sets for a CJM based on a human factors approach. The 10‐step process and the rule sets were built on case studies of 25 categories of mobile services. Large‐scale case studies were conducted with mobile service providers and combined with the result of a user diary method that collected users’ daily activities and the difficulties that the user perceived when performing a task. We suggested various methods for using the CJM to generate new service opportunities. The proposed design process and the way for opportunity discovery can help service designers to develop unified CJMs and to identify innovative service ideas.