PURPOSE: To design a servo system corresponding to H∞ partial state feedback control and to turn offset to '0' even when input disturbance exists or the integration characteristic of a controlled system is changed.

CONSTITUTION: At the H∞ control designing device applied to the design of an automatic control system, a generalized plant is generated by inputting sensitivity designated by a sensitivity designating means 12 and the controlled system from a controlled system input means 11. This generalized plant is made discrete by performing Z transformation with 0th-order hold at a generalized plant discrete means 14, and this discrete generalized plant is returned to continuous areas by performing bilinear transformation at a generalized plant continuous area transforming means 15. Then, A partial state feedback solution corresponding to this generalized plant returned to the continuous areas is calculated by a partial state feedback control solution generating means 16 and further, that solution is inputted to a digital controller calculating means 17 so as to lead out a digital controller.