A sound signal processing method according to an embodiment includes a step of acquiring an input sound signal, a step of acquiring a beat number per unit time period from the input sound signal, a step of normalizing the input sound signal with the beat number per unit time period, a step of calculating a beat spectrum of the normalized input sound signal, and a step of calculating a rhythm similarity between the beat spectrum of the normalized input sound signal and a normalized beat spectrum calculated from a reference sound signal.