Recently mobile phones have included new functions to provide new services for users. However, increase in functions complicates mobile phones. This is a serious problem in developing mobile phones. Since built-in applications on mobile phones provide the main functions of mobile phones, productivity of built-in applications has to be improved. The Java language improves productivity of applications because of the features of the Java language. To develop built-in applications as Java applications, an application platform should handle Java applications and native applications in a uniform way. In this paper, we propose an application platform for mobile phones which enables running native applications and Java applications concurrently and handling them in a uniform way. To handle applications in a uniform way, we developed a platform which has window management, inter-application communication and application state management functions.