In the design of a mechanical component for a mechanical device driven by a feedback controlled electric motor, the user is enabled to easily know how the properties of mechanical component affect the generation of abnormal vibrations of the mechanical device. In a design assist apparatus (1), the processor (11) is configured to set a plurality of parameters of a mathematical model of an analysis target component selected from one or more mechanical components (24, 56, 58) forming the mechanical device, compute a pole of a transfer function of the mechanical device associated with one or more vibration modes of the mechanical device according to the parameters, and create a stability determination diagram including an isoline of a real part of the pole of the transfer function.