In a signal encoding apparatus (1) a frequency normalization unit (11) normalizes each spectrum of spectral signals by using respectively normalization factors and supplies a normalization factor index per spectrum to a quantization accuracy determining unit (13). The quantization accuracy determining unit (13) adds a weighting factor using auditory properties to the normalization factor index per spectrum of range conversion spectral signals which are subjected to normalization as well as range conversion, and the quantization accuracy is determined according to the result of addition. Then, a quantization unit (14) performs quantization with the quantization accuracy corresponding to a quantization accuracy index supplied from the quantization accuracy determining unit (13), while the encoding/code string generating unit (15) encodes the weighting factor supplied from the quantization accuracy determining unit (13), together with the normalization factor index and the quantized spectral signal.