We propose a method for separating accompaniment from polyphonic music and its karaoke application, both based on automatic melody transcription. First, the method transcribes the lead-vocal melody of an existing polyphonic music piece, where the transcription consists of a MIDI note sequence and a detailed fundamental frequency (F0) trajectory for each note. Based on the note F0 trajectories, the method uses sinusoidal modeling to estimate, synthesize, and remove the lead vocals in the piece, thus producing separated accompaniment of the piece. User sings along with the separated accompaniment similar to karaoke while the user singing can be tuned to the transcribed melody. This will help non-professional singers to produce more appealing karaoke performances. The quality of separated accompaniments was quantitatively evaluated with approximately one hour of polyphonic music, including material from a commercial karaoke DVD.