PURPOSE:To enable to increase the efficiency of an air turbine, by obtaining stable pneumatic energy from wave energy. CONSTITUTION:Air compressed to a required pressure is accumulated in an air tank 51 by converting wave energy into pneumatic energy and driving reversible turbines 46 and air compressors 48 by the pneumatic energy. As compared with a wave energy the level of which is varied in a short period, the level of pneumatic energy is smoothed in the air tank 51. The energy level being smoothed, a compressed air is supplied to a turbine 55 at a constant rate under the control of a flow control valve 58. Therefore, an optimal amount of air is supplied in a stable manner to the main air turbine 55, so that it can be operated at a high efficiency.