PURPOSE:To convert a gain index by code excitation linear prediction encoding to a low bit. CONSTITUTION:The device is provided with an RMS calculating circuit 304 for predicting amplitude of a statistical excitation vector by deforming the statistical excitation vectors based on an RMS(Root Mean Square) of an adaptive excitation vector, and a multiplier 306. Also, this device is provided with a VQ gain code book 305 obtained by vector-quantizing statistical excitation gain and adaptive excitation gain. According to this constitution. with regard to an excitation gain index, even if it is converted to a low bit, an excellent composite voice quality is obtained.