This paper describes embedding explanation mechanism within a UIMS called the G-system, which being developed at Yamanashi University. A UIMS is an application independent user interface management system, which can serve as the user interface for a variety of functional system. One of the important facilities of the G-system is explanation facility. This facility provides online assistance of applications in the G-system via which users can obtain a consistent assistance that display summary information, command descriptions, explanation of error messages and other online documentation. The use of the explanation facility and knowledge database provides a common interface between many type of applications and user interface system, and separates the functions of application and user interaction.