The radiation relay is used for the measurment and control where a ordinary type level meter is not available. It is adequate for the control of the levels of melted metal, liquid in high pressure tank and etc… In this system the controlled object is placed beween a radiation source and detector. As the objct with uniform thickness covers the window of detector from one side to the other with uniform speed, the amout of incident radiation is reduced. At the first time when the output of detector reaches the given threshold, a relay acts and the motion of the object is stopped. Haw to design the system obtaining the given value of the mean stopping position and minimize the standard deviation of it is presented in this paper.