A signal processing device comprising: an acquisition unit configured to acquire sound data subjected to quantization, and configured to cut out a time waveform from the sound data for each window width with a predetermined time length, frame, while applying a window function with overlap, and to provide the sound data for each frame to each of a conversion unit and an quantization noise control unit (23, 24); and the conversion unit configured to perform fast Fourier transform, FFT, on the sound data with the predetermined time length received from the acquisition unit, and to output for each frequency information on an amplitude to the quantization noise control unit (23, 24) and an information on the phase to an overtone generation unit; the quantization noise control unit (23, 24) configured to determine a control amount for quantization noise of the sound data, generated due to the quantization, based on the level of the sound data, the quantization noise control unit (23, 24) configured to: determine a maximum attenuation amount with respect to the amplitude in a frequency domain to be attenuated based on the magnitude of the signal level of sound data received from the acquisition unit; adjust the amplitude by an attenuation amount corresponding to the control amount, in a frequency band that a sound level of the sound data is less than a predetermined threshold so as not to exceed the maximum attenuation amount to attenuate the quantization noise; the overtone generation unit configured to generate an overtone for high-frequency interpolation based on information on the amplitude for each frequency received from the quantization noise control unit (23, 24), and information on the phase received from the conversion unit, an inverse fast Fourier transform, IFFT, block configured to perform inverse fast Fourier transform on the sound data in the frequency domain subjected to the overtone generation processing, to convert the sound data for each frame from the frequency domain to a time domain, and an output unit configured to provide an output signal by overlapping and adding sound data of each frame outputted by the IFFT block. an acquisition unit configured to acquire sound data subjected to quantization, and configured to cut out a time waveform from the sound data for each window width with a predetermined time length, frame, while applying a window function with overlap, and to provide the sound data for each frame to each of a conversion unit and an quantization noise control unit (23, 24); and the conversion unit configured to perform fast Fourier transform, FFT, on the sound data with the predetermined time length received from the acquisition unit, and to output for each frequency information on an amplitude to the quantization noise control unit (23, 24) and an information on the phase to an overtone generation unit; the quantization noise control unit (23, 24) configured to determine a control amount for quantization noise of the sound data, generated due to the quantization, based on the level of the sound data, the quantization noise control unit (23, 24) configured to: determine a maximum attenuation amount with respect to the amplitude in a frequency domain to be attenuated based on the magnitude of the signal level of sound data received from the acquisition unit; adjust the amplitude by an attenuation amount corresponding to the control amount, in a frequency band that a sound level of the sound data is less than a predetermined threshold so as not to exceed the maximum attenuation amount to attenuate the quantization noise; determine a maximum attenuation amount with respect to the amplitude in a frequency domain to be attenuated based on the magnitude of the signal level of sound data received from the acquisition unit; adjust the amplitude by an attenuation amount corresponding to the control amount, in a frequency band that a sound level of the sound data is less than a predetermined threshold so as not to exceed the maximum attenuation amount to attenuate the quantization noise; the overtone generation unit configured to generate an overtone for high-frequency interpolation based on information on the amplitude for each frequency received from the quantization noise control unit (23, 24), and information on the phase received from the conversion unit, an inverse fast Fourier transform, IFFT, block configured to perform inverse fast Fourier transform on the sound data in the frequency domain subjected to the overtone generation processing, to convert the sound data for each frame from the frequency domain to a time domain, and an output unit configured to provide an output signal by overlapping and adding sound data of each frame outputted by the IFFT block.