A fixed oscillating water column (OWC)-type wave energy converter is composed of an air chamber, an air turbine and a generator. In this unit, the energy conversion process is divided into two main steps, the primary conversion in an air chamber and the secondary conversion in an air turbine. For the practical application of a fixed OWC-type wave energy converter, it is necessary to develop a design method which can consider the characteristics of incident wave motion, the motion of the internal free surface affected in the structure such as a curtain wall, the fluctuation of air pressure in an air chamber, and the rotation of the air turbine. On the other hand, the impulse turbine as the secondary conversion device in the OWC unit is expected to achieve high efficiency throughout a wide flow rate range. The authors conducted the 2-dimensional wave tank tests by means of the model OWC equipped with the impulse turbine to obtain the data needed to make this design method. In past studies, the effects of air chamber length, turbine geometry and curtain wall depth on energy conversion efficiency were investigated experimentally. In this paper, the nozzle geometry of air chamber is changed, and its performance is discussed.