For a mechanical device that is provided with a motor as a drive source for an operating shaft, this failure diagnosis device acquires, at prescribed periods, a movement position for the operating shaft and a value for the disturbance torque applied to the operating shaft and makes a diagnosis of failure when an acquired disturbance torque value is greater than a failure-determination threshold value. The failure diagnosis device also calculates the variation of acquired disturbance torque values from respective reference values and, excepting disturbance torque values for which the variationfrom said reference values is at or above a prescribed threshold value, stores the disturbance torque values.