Provided is abnormality diagnosis with improved stability on an appliance to which a rotational torque is transmitted from a rotating body. A state monitoring device monitors the state of an appliance to which the rotational torque of a rotating body is transmitted. The state monitoring device includes a storage unit and a controller. Specific information depending on a rotational speed of the rotating body, produced from vibration data of the appliance, is pre-stored in the storage unit. Using the rotational speed of the rotating body and the vibration data of the appliance, the controller performs abnormality diagnosis on the appliance. Using the specific information, the controller estimates the rotational speed of the rotating body, the rotational speed being a rotational speed when the vibration data of the appliance is measured.