1. A monitoring device comprising: a unit-specific waveform data acquisition unit that acquires unit-specific monitoring waveform data which is waveform data of at least one among total current consumption, a total input voltage, and total power consumption in a unit in which monitoring target electrical devices are installed; a first inference unit that infers operation states of at least some of the monitoring target electrical devices based on a 1st feature amount group including at least one kind of feature amount extracted from the unit-specific monitoring waveform data, and a training feature amount which is a feature amount of each of the monitoring target electrical devices in a predetermined operation state; and a second inference unit that infers the operation states of some of the monitoring target electrical devices based on a 2nd feature amount group including at least one kind of feature amount extracted from the unit-specific monitoring waveform data, and different from the 1st feature amount group, and the training feature amount.