This paper proposes a discrete optimal control method based on a multi-rate digital observer for multi sensor network systems. In many multi sensor network systems, the sensing time ratios are generally different each other such that the identical estimation time should be applied at the fastest measuring time for design of an observer in spite of the repeatability of output signals sensed at slower measuring time. This paper proposes the optimal control method based on Kalman filter to store repeated signals in memories in order avoid unnecessary processes. The proposed control system is operated according to the slow measuring time. All signals are calculated to estimate the system states without any repeated data. The proposed design method of the discrete observer develops the performance of low level microcontroller with slow operating abilities since the controller is able to actuate at slow operating time without any loss of output information using simple data storage systems. The optimal stability of the controlled system is verified according to the separation principle of linear quadratic Gaussian system with a single time ratio.