This paper introduces jPop-E (java-based PolyPhrase Ensemble), an assistant system for the Pop-E performancerendering system. Using this assistant system, MIDI dataincluding expressive tempo changes or velocity control canbe created based on the user’s musical intention. Pop-E(PolyPhrase Ensemble) is one of the few machine systemsdevoted to creating expressive musical performances thatcan deal with the structure of polyphonic music and theuser’s interpretation of the music. A well-designed graphical user interface is required to make full use of the potential ability of Pop-E. In this paper, we discuss the necessaryelements of the user interface for Pop-E, and describe theimplemented system, jPop-E.