To highly accurately encode an audio signal including noise in a prescribed bandwidth.

A noise detection unit 51 detects noise unique to a PDM signal based on an audio signal. When the noise unique to the PDM signal is detected, a gain adjustment unit 52 adjusts the gain of the audio signal so as to attenuate a high-frequency component outside the audible band of the audio signal. A bit allocation calculation unit 13 calculates a bit amount to be allocated to a normalized frequency spectrum nspec based on normalized information idsf of the audio signal after the gain adjustment. A quantization unit 14 quantizes the normalized frequency spectrum nspec based on the quantization information idwl representing the bit amount. This invention can be applied to, for example, an audio encoder.