Up-sampler generates an up-sampled sound signal from the sound signal. From the up-sampled sound signal, odd-ordered high-harmonic generator generates an odd-ordered high-harmonic, and even-ordered high-harmonic generator generates an even-ordered high-harmonic. Vowel sound detector identifies whether or not the sound signal is vowel sound, and generates a first gain value and a second gain value. First gain controller amplifies or attenuates the odd-ordered high-harmonic based on the first gain value, and outputs the resultant odd-ordered high-harmonic. Second gain controller amplifies or attenuates the even-ordered high-harmonic based on the second gain value, and outputs the resultant even-ordered high-harmonic. Sound signal processing device adds the gain-adjusted odd-ordered high-harmonic and the gain-adjusted even-ordered high-harmonic to the up-sampled sound signal, and outputs the up-sampled sound signal having the high-harmonics added.