Provided is a control device capable of automatically determining whether or not an inertia estimation function needs to be activated. The control device 10 is for an electric motor and comprises: a first inertia estimation unit 11 that estimates whether or not there has been a change in the inertia of an object to be driven, on the basis of at least one among first information pertaining to an operation program or operation settings for a device comprising the electric motor, second information obtained from a detection device for detecting the shape of the object to be driven by the electric motor, and third information indicating the operation state of the electric motor; and a second inertia estimation unit 12 that estimates the inertia of the object to be driven if the first inertia estimation unit 11 has estimated that there has been a change in the inertia of the object to be driven.