1. A classification apparatus comprising: a memory that stores a phenomenon relating to a situation around a device on which a sensor is mounted and an image of a two-dimensional Fourier transform of a spectrogram of a time variation signal detected in relation to the phenomenon, as a template image, to be associated with each other; and a processor acting as: a spectrogram generation unit that generates the spectrogram of the time variation signal of a classification object by processing the time variation signal of the classification object obtained by the sensor; a two-dimensional Fourier transform calculation unit that calculates the two-dimensional Fourier transform of the spectrogram generated by the spectrogram generation unit; a similarity calculation unit that calculates a similarity between the template image and an image of the two-dimensional Fourier transform obtained by the two-dimensional Fourier transform calculation unit, for each template image corresponding to each phenomenon stored in the template image memorizing unit; a determination unit that determines whether the time variation signal of the classification object conforms to any of one or more phenomena, on the basis of the similarity calculated by the similarity calculation unit; a characteristic memorizing unit that stores periodicity characteristics included in the two-dimensional Fourier transform of the spectrogram of the time variation signal detected in relation to the phenomenon, as a template characteristics, with regard to each of one or more phenomena; a characteristic detection unit that detects the periodicity characteristics from an image of the two-dimensional Fourier transform obtained by the two-dimensional Fourier transform calculation unit; and a specification unit that specifies one similar to the periodicity characteristics detected by the characteristic detection unit, from among the template characteristics stored in the characteristic memorizing unit, wherein the determination unit determines that the time variation signal of the classification object conforms to the phenomenon corresponding to the template characteristics specified by the specification unit, in a case where the template characteristics similar to the periodicity characteristics detected by the characteristic detection unit is specified by the specification unit, and wherein in a case where the template characteristics similar to the periodicity characteristics detected by the characteristic detection unit is not specified by the specification unit, the similarity calculation is performed by the similarity calculation unit.