A musical tone signal is synthesized based on performance information to simulate a sound generated from a musical instrument having a string and a body that supports the string by a support. There is provided a closed loop circuit having a delay element that simulates delay characteristic of vibration propagated through the string and a characteristic control element that simulates a variation in amplitude or frequency. A string model calculation circuit inputs an excitation signal based on the performance information to the closed loop circuit, and calculates first information representing a force of the string acting on the support based on a cyclic signal generated in the closed loop and representing the vibration of the string circuit. A body model calculation circuit calculates second information representing a displacement of the body or a derivative of the displacement. A musical tone signal calculation circuit calculates the musical tone signal.