PURPOSE: To increase the number of pulses in the whole frame and to improve sound quality by allocating the smaller number of pulse amplitude quantizing bits for several pulses in the latter half of a frame as compared with pulses other than the latter half of the frame.

CONSTITUTION: When the number of pulses per frame is 15 pulses at the time of uniformly applying 4-bit quantization to all pulses found out through pulse search by a quantizer, the number of pulse amplitude quantizing bits per frame becomes 4×15=60 bits in the allocation of pulse amplitude quantizing bits. When the number of amplitude quantizing bits per frame is set to 60 bits, 12 pulses in the first half is 4-bit quantized by the quantizer and 4 pulses in the latter half of the frame is 3-bit quantized by a quantizer for faithfully quantizing only a range having small pulse amplitude, one pulse can be added to the latter half of the frame and the number of pulses per frame becomes 16 pulses. Consequently sound quality can be improved.