Musical performance with digital instruments has become a common practice today and many digital instruments came to be used in Desk Top Music, live performance, etc. However, the spread of digital contents causes the problem of illegal duplication and distribution, so that digital watermark has recently attracted much attention as a technology to solve this problem. In this project we focus on a sound synthesized process in digital instruments and propose a real-time watermark method. Certain watermarks are embedded in wavetables that include in our digital instruments and the insertion of secret messages is actualized with wave table switching additionally embedded watermarks can be extracted from the acoustic signal. The proposed method is able to achieve a real-time watermark.