In this paper, we propose a new method for implementation of a High-Gain Observer (HGO) to cope with measurement noise through Stochastic Approximation (SA). Most approaches have been using a trade-off between a speed of regeneration of states in the presence of uncertainties and the effect of measurement noise. Through SA, we are able to reserve authentic properties of the HGO i.e., a fast reconstruction of system states and robustness to uncertainties, while dealing with the effect of measurement noise both in practical and analytical manners. Regularity conditions are verified to analyze the stability by the associated Ordinary Differential Equation (ODE), i.e., the ODE method. Stability analysis of the full system is conducted. Numerical simulation results demonstrate the validity of the proposed design method.