In this paper, a control method with which it is possible to extract wind energy effectively, even when the system characteristic is unknown, is suggested. With this control method, it is possible to follow the quasi-maximum output only by measurements of the load terminal voltage and current, which are relatively simple to measure with high accuracy. The hill-climbing method, among adaptive control methods, was adopted. For the purpose of confirming the effectiveness of this method, investigation by simulation using a model of a small-scale windmill generating system with a resistive load was conducted.