There is provided a signal processing device for processing an audio signal, the signal processing device including: an onset time detection unit for detecting an onset time based on a level of the audio signal; and a beat length calculation unit for obtaining a beat length Q by: setting an objective function P(Q|X) and an auxiliary function, the objective function P(Q|X) representing a probability that, when an interval X between the onset times is given, the interval X is the beat length Q, the auxiliary function being for inducing an update of both the beat length Q and a tempo Z that results in a monotonous increase of the objective function P(Q|X); and repeating maximization of the auxiliary function to have the auxiliary function converge.