This paper presents a novel method for designing output feedback servo-controllers for nonlinear systems. First, a design method of state feedback regulators for nonlinear systems is derived based on the Hamilton-Jacobi optimality conditions for cost functions with quadratic structure. Second, observers of nonlinear systems are designed, and the output feedback servo-controller is designed by combining the regulator and observer. The validity of the proposed method has been verified through simulations using a two link robot arm.