A signal processing device, comprising: a band detecting means for detecting a frequency band which satisfies a predetermined condition from an audio signal; a reference signal generating means for generating a reference signal in accordance with a detection band by the band detecting means; a reference signal correcting means for correcting the reference signal generated by the reference signal generating means to a flat frequency characteristic on a basis of a frequency characteristic of the generated reference signal; a frequency band extending means for extending the corrected reference signal up to a frequency band higher than the detection band; an interpolation signal generating means for generating an interpolation signal by weighting each frequency component within the extended frequency band in accordance with a frequency characteristic of the audio signal; and a signal synthesizing means for synthesizing the generated interpolation signal with the audio signal; wherein the interpolation signal generating means is configured to perform a first regression analysis on at least a portion of the audio signal; calculate an interpolation signal weighting value for each frequency component within the extended frequency band on a basis of a slope of a frequency characteristic of the at least a portion of the audio signal obtained by the first regression analysis; and generate the interpolation signal by multiplying the calculated interpolation signal weighting value for each frequency component and each frequency component within the extended frequency band together; and characterized in that at least one of: the interpolation signal generating means increases the interpolation signal weighting value as the slope of the frequency characteristic of the at least a portion of the audio signal gets greater in a minus direction; and the interpolation signal generating means increases the interpolation signal weighting value as an upper frequency limit of a range for the first regression analysis gets higher. a band detecting means for detecting a frequency band which satisfies a predetermined condition from an audio signal; a reference signal generating means for generating a reference signal in accordance with a detection band by the band detecting means; a reference signal correcting means for correcting the reference signal generated by the reference signal generating means to a flat frequency characteristic on a basis of a frequency characteristic of the generated reference signal; a frequency band extending means for extending the corrected reference signal up to a frequency band higher than the detection band; an interpolation signal generating means for generating an interpolation signal by weighting each frequency component within the extended frequency band in accordance with a frequency characteristic of the audio signal; and a signal synthesizing means for synthesizing the generated interpolation signal with the audio signal; wherein the interpolation signal generating means is configured to perform a first regression analysis on at least a portion of the audio signal; calculate an interpolation signal weighting value for each frequency component within the extended frequency band on a basis of a slope of a frequency characteristic of the at least a portion of the audio signal obtained by the first regression analysis; and generate the interpolation signal by multiplying the calculated interpolation signal weighting value for each frequency component and each frequency component within the extended frequency band together; and characterized in that at least one of: the interpolation signal generating means increases the interpolation signal weighting value as the slope of the frequency characteristic of the at least a portion of the audio signal gets greater in a minus direction; and the interpolation signal generating means increases the interpolation signal weighting value as an upper frequency limit of a range for the first regression analysis gets higher. perform a first regression analysis on at least a portion of the audio signal; calculate an interpolation signal weighting value for each frequency component within the extended frequency band on a basis of a slope of a frequency characteristic of the at least a portion of the audio signal obtained by the first regression analysis; and generate the interpolation signal by multiplying the calculated interpolation signal weighting value for each frequency component and each frequency component within the extended frequency band together; and characterized in that at least one of: the interpolation signal generating means increases the interpolation signal weighting value as the slope of the frequency characteristic of the at least a portion of the audio signal gets greater in a minus direction; and the interpolation signal generating means increases the interpolation signal weighting value as an upper frequency limit of a range for the first regression analysis gets higher. the interpolation signal generating means increases the interpolation signal weighting value as the slope of the frequency characteristic of the at least a portion of the audio signal gets greater in a minus direction; and the interpolation signal generating means increases the interpolation signal weighting value as an upper frequency limit of a range for the first regression analysis gets higher.