This paper presents a beat tracking system that processes acoustic signals of music and recognizes temporal positions of beats in real time. Musical beat tracking is needed by various multimedia applications such as video editing, audio editing, and stage lighting control. Previous systems were not able to deal with acoustic signals that contained sounds of various instruments, especially drums. They dealt with either MIDI signals or acoustic signals played on a few instruments, and in the latter case, did not work in real time. Our system deals with popular music in which drums maintain the beat. Because our system examines multiple hypotheses in parallel, it can follow beats without losing track of them, even if some hypotheses become wrong. Our system has been implemented on a parallel computer, the Fujitsu AP1000. In our experiment, the system correctly tracked beats in 27 out of 30 commercially distributed popular songs.