In this paper, we propose a method for a transcription system for music by two instruments. We analyze all of the main elements printed on the score, including tempo, time of attack, tone, pitch and length from the sound. The method is mainly time-frequency analysis. This transcription system is for monaurally recorded music and the instruments we cover in this system are piano, flute, trumpet and violin. We simulated the system with one instrument, monophony and two instruments chosen arbitrarily from the four instruments, polyphony. The accuracy depended on the combination of the instruments, but a considerably high accuracy was obtained in various patterns.