PURPOSE: To provide a system which easily makes contrast adjustment against the power value that is held by formant and outputs a spectrogram that clearly indicates formant.

CONSTITUTION: The power value extracted from a spectrogram is mapped to an area value in accordance with the output value of the control function registered in a function register 9 and a contrast graph, which corresponds to the power value, is obtained. The control function is a synthesized sigmoid function, which is made of two sigmoid functions in that parameters are individually set, connected at an inflection point and is generated in a function synthesis section 8. In order to perform a contrast adjustment of formant to fit with the usage, inclination of each slope curve and parameters of threshold value of the synthesized sigmoid function are arbitrary updated.