Servo control is one of foundamental control techniques for control problems of many mechanical structures. Various kinds of control methods such as PID, LQ and H-infinity controls have been applied to active servo control designs. However, it is very important to guarantee against safety in the control problems of man-machine systems. Especially, driving recklessly due to software or hardware errors in active control systems must be avoided in some devices, e.g., nursing care or medical devices. Therefore, an intrinsically safe control system is required for the devices. This note discusses the use of an intrinsically safe servo control system consisting of passive elements instead of active actuators. For such a control system, semi-active control to vary passive parameters such as viscous damping is important. Therefore, a novel servo control method based on the bilinear optimal control theory is proposed for the system in this note. The effectiveness of the method is verified by simulations.