This paper is concerned with the design of a gain-scheduled controller with guaranteed L2 performance for the water level control of a nuclear steam generator which has parameter varying dynamics. The proposed controller guarantees both stability and a prescribed L2 performance in the entire operating region. The systematical design method for the controller which is automatically adjusted along the reactor power, in terms of Linear Matrix Inequalities (LMIs), is shown. The main benefit of this method is to avoid most of difficulties such as gain-interpolation or griding technique in a classical gain scheduling control. The computer simulation demonstrates the effectiveness of the proposed controller in full operating range and its superiority to conventional PID controllers.