This paper is concerned with the sampled-data control of a continuous system in the situation where there exists delay for the control action after the measurement of the output at each sampling instant. The optimal control with the quadratic criterion function defined for the continuous system is adopted in this control problem. This paper shows that the problem can be transformed into the optimal control of a discrete system for the quadratic criterion function, and the variation of the control law to the sampling period and to the delay of the control action is clarified. The deterioration of cost performance with respect to delay is derived. A numerical example is also presented.