A signal processing device for processing an input signal includes gain calculating means and feature quantity calculating means. The gain calculating means is configured to obtain information indicating magnitude of noise to be added to the input signal on a basis of periodicity information indicating periodicity of the input signal and power of the input signal. The feature quantity calculating means is configured to obtain periodicity information of a noise-added signal obtained by adding noise having magnitude corresponding to the gain information to the input signal as a feature quantity of the input signal.