Monitoring Device

A monitoring device includes a vibration-driven energy harvesting element as a vibration-driven energy harvesting unit that performs power generation in response to a sound or a vibration being occurred by a monitoring target and having a predetermined frequency, and a notification unit that notifies a state change of the monitoring target based on a magnitude relationship between a predetermined threshold value and a voltage value of the power generation by the vibration-driven energy harvesting element. The notification unit is configured to notify the state change when the vibration-driven energy harvesting element performs the power generation. Therefore, the vibration-driven energy harvesting element, which functions as a sensor unit, requires no power supply.