A vocoder system for improving the performance expression of an output sound while lightening the computational load. The system includes formant detection means and division means in which the center frequencies have been fixed. The modulation level with which the levels of each of the frequency bands that have been divided in the division means are set by a setting means based on the levels of each of the frequency bands that correspond to those that have been detected in the formant detection means and formant information with which the formants are changed. Therefore, it is possible to improve the performance expression of the output sound with a light computational load and without the need to calculate and change the filter figure of each filter for each sample in order to change the center frequency and bandwidth of each of the filters comprising the division means.