A decoding device (600, 610, 620, 700) comprising: a separating unit that separates first encoded data, where a spectrum including a low-band spectrum of audio signals has been encoded, and second encoded data where a high-band spectrum of a higher band than the low-band spectrum has been encoded, based on the first encoded data; a first decoding unit that decodes the first encoded data and generates a first decoded spectrum; a first amplitude normalizer that divides the amplitude of the first decoded spectrum into a plurality of sub-bands, normalizes the spectrum of each sub-band by the largest value of the amplitude of the first decoded spectrum within each sub-band, and generates a normalized spectrum; a noise generating unit that generates a noise spectrum; a noise amplitude normalization unit that normalizes the noise spectrum and outputs a normalized noise spectrum; a noise spectrum amplitude adjustment unit that adjusts an amplitude of the normalized noise spectrum to obtain an adjusted normalized noise spectrum; an addition unit that adds the adjusted normalized noise spectrum to an adjusted normalized spectrum and generates a noise-added normalized spectrum; a second decoding unit that decodes the second encoded data using the noise-added normalized spectrum, and generates a second noise-added spectrum; and a converter that performs a conversion into a time domain regarding a spectrum generated by concatenating a spectrum based on the first decoded spectrum and a spectrum based on the second noise-added spectrum, characterized by a threshold value calculating unit that calculates a threshold value of spectral intensity that separates a noise component from a non-noise component, using sparse information indicating a degree of sparseness of the normalized spectrum or of the first decoded spectrum; wherein the noise spectrum amplitude adjusting unit performs adjustment of the amplitude of the normalized noise spectrum so that the largest value of the normalized noise spectrum is equal to the threshold value or lower; and a first decoded spectrum amplitude adjustment unit that adjusts the amplitude of the normalized spectrum so that a non-zero component of the normalized spectrum is larger than the threshold value to obtain the adjusted normalized spectrum. a separating unit that separates first encoded data, where a spectrum including a low-band spectrum of audio signals has been encoded, and second encoded data where a high-band spectrum of a higher band than the low-band spectrum has been encoded, based on the first encoded data; a first decoding unit that decodes the first encoded data and generates a first decoded spectrum; a first amplitude normalizer that divides the amplitude of the first decoded spectrum into a plurality of sub-bands, normalizes the spectrum of each sub-band by the largest value of the amplitude of the first decoded spectrum within each sub-band, and generates a normalized spectrum; a noise generating unit that generates a noise spectrum; a noise amplitude normalization unit that normalizes the noise spectrum and outputs a normalized noise spectrum; a noise spectrum amplitude adjustment unit that adjusts an amplitude of the normalized noise spectrum to obtain an adjusted normalized noise spectrum; an addition unit that adds the adjusted normalized noise spectrum to an adjusted normalized spectrum and generates a noise-added normalized spectrum; a second decoding unit that decodes the second encoded data using the noise-added normalized spectrum, and generates a second noise-added spectrum; and a converter that performs a conversion into a time domain regarding a spectrum generated by concatenating a spectrum based on the first decoded spectrum and a spectrum based on the second noise-added spectrum, characterized by a threshold value calculating unit that calculates a threshold value of spectral intensity that separates a noise component from a non-noise component, using sparse information indicating a degree of sparseness of the normalized spectrum or of the first decoded spectrum; wherein the noise spectrum amplitude adjusting unit performs adjustment of the amplitude of the normalized noise spectrum so that the largest value of the normalized noise spectrum is equal to the threshold value or lower; and a first decoded spectrum amplitude adjustment unit that adjusts the amplitude of the normalized spectrum so that a non-zero component of the normalized spectrum is larger than the threshold value to obtain the adjusted normalized spectrum.