A wave activated power generation device includes a cylindrical floating body anchored on water and extending vertically to a water surface. a ba last provided at a lower end portion of the floating body and a power generating equipment provided at an upper end portion of the floating body The floating body includes a buoyancy control chamber allowing the floating body to vertically float in the water and adjusting a position of the floating body relative to the water surface to a predetermined height in accordance with a relation between buoyancy of the floating body and weight of the ballast, ad an air chamber formed between the buoyancy control chamber and the power generating equipment. The power generating equipment includes an air turbine provided above the air chamber and driven by air flow generated between a inside and outside of the air chamber by vertical motion of the water surface, ad a generator rotationally driven by the air turbine. Further ore, a wave activated power generation plant includes the coupled plural wave activated power generation devices upon request.