PURPOSE: To improve quantization characteristics without increasing the transmission bit rate.

CONSTITUTION: When code exciting linear prediction(CELP) is coded, a parameter α is taken out of an inputted speech signal by a linear prediction code(LPC) analyzing circuit 12, and is converted into a line spectrum pair(LSP) parameter by an α → LSP converting circuit 13, and vector quantized by a vector quantization unit 14. At this time, a changeover switch 16 is controlled in a switching system according to a value of a pitch detected by a pitch detecting circuit 22, and either one of a code book 15M for a male voice and a code book 15F for a female voice is selectively used.