A monitoring device (10) includes a unit-specific waveform data acquisition unit (11) that acquires waveform data in a unit in which electrical devices are installed; a first inference unit (13) that infers change in operation states of at least some of the electrical devices based on a first monitoring difference group including at least one of at least one kind of feature amount extracted from waveform data of a difference between waveform data of a first timing and waveform data of a second timing in the waveform data, and a differences of at least one kind of feature amounts extracted from the waveform data of the first timing and the waveform data of the second timing, and training difference information regarding a difference between a first operation state and a second operation state of each of the electrical devices; and a second inference unit (14) that infers an operation state of each of the electrical devices based on an inference result of the first inference unit (13).