A power generation apparatus (1, 1a) using wave energy conversion by gravity includes a sealed body (10, 10a), a rotor (20, 20a), a gear box (30,30a), a power generation motor (40, 40a), and a pontoon (50, 50a). The rotor (20, 20a), disposed in the sealed body (10, 10a), includes an eccentric disk (21, 21a), a plurality of rollers (22, 22a) and a rotating shaft (23, 23a) penetrating through the eccentric disk (21, 21a). A center of gravity of the eccentric disk (21, 21a) is disposed with an offset to that of the sealed body (10, 10a). The gear box (31) is driven by the rotating shaft (23, 23a). The power generation motor (40, 40a) is driven by the gear box (31) to rotate and generate electric power.