a frame dividing unit configured to divide an audio input signal into processing unit frames for respective channels;

a psychoacoustic arithmetic unit configured to analyze the audio input signal to determine a transform block length and to make an auditory masking calculation;

a filter bank unit configured to decompose a frame to be processed into blocks in accordance with the transform block length determined by said psychoacoustic arithmetic unit to transform time domain signals in the frame into one or more sets of frequency spectrum;

a scale factor calculation unit configured to divide the frequency spectrum output from said filter bank unit into a plurality of frequency bands, and calculate scale factors for weighting the spectrum in the respective frequency bands based on an arithmetic result of said psychoacoustic arithmetic unit;

a quantization step determination unit configured to determine a quantization step of the entire frame prior to spectrum quantization by subtracting an information amount of all quantized spectrum from a perceptual information amount of all the spectrum before quantization, which are weighted by the scale factors calculated by said scale factor calculation unit, and multiplying the difference by a coefficient obtained from a step width of a quantization coarseness;

a spectrum quantization unit configured to quantize the frequency spectrum using the scale factors and the quantization step; and

a bit shaping unit configured to form and output a bitstream obtained by shaping quantized spectrum output from said spectrum quantization unit in accordance with a predetermined format,

wherein said quantization step determination unit includes a quantized spectral information amount prediction unit configured to predict the information amount of all the quantized spectrum based on a bit size assigned to a frame to be encoded.