The invention concerns an apparatus for converting wave energy into electric power, comprising a closed floating element (1) and a mass forming element (2), the mass forming element (2) being arranged inside the floating element (1) and mounted mobile relative to the floating element (1), the mass forming element (2) being adapted to be set in relative motion with respect to the floating element (1) under the action of the waves on the floating element (1). The invention is characterized in that it also comprises locking means (13) adapted to lock the movement of the mass forming element (2) and control means adapted to control the locking means (13) for selectively locking or releasing the mass element (2) so as to amplify the movement of the mass forming element (2) by constant adaptation to the dynamics of the apparatus to successive waves.