An object of the present invention is to preferably detect the failure accompanied with the decrease in the pre-load in a ball screw unit. The failure diagnosis is performed for the ball screw unit on the basis of a combination of a resonance frequency in an axial direction of a predetermined machine assembly including the ball screw unit and a motor and a position of a nut member on a screw shaft corresponding to the resonance frequency as provided when the screw shaft is driven and rotated by the motor.