The present invention relates to a pendulum electricity-generating device using natural energy, which is installed on the seashore, and which can generate electric energy stably and efficiently according to the flow on the sea surface. The present invention comprises: a housing that floats on the sea surface using buoyancy; buoyancy wings integrally connected to both sides of the housing so as to rotate the housing according to the floating movement of the sea surface; a power unit installed inside the housing in the longitudinal direction thereof; and a gravity body installed in the longitudinal direction of the power unit so as not to rotate in conformity with rotation of the housing, thereby transferring the rotational force to the power unit according to the floating movement of the housing. The present invention, described above, has the following advantages: firstly, even when installed and used at a place where the sea surface does not undergo a sufficient floating movement, stable electricity generation can be guaranteed while flexibly dealing with the horizontal flow of waves or the vertical flow of waves. Secondly, easy fracturing can be prevented even in the case of an abrupt floating movement of the sea surface; the characteristic feature, which is for the purpose of generating electricity using natural energy, is provided inside the housing such that, by effectively preventing corrosion of the constituent elements of the electricity-generating device by the salt in the seawater, the service life is increased, thereby reducing the maintenance cost; and configuration of multiple pendulum generators can maximize the efficiency of electricity generation. Thirdly, multiple generation units are installed and configured on the outside of the power unit so as to transfer the rotational force to the power unit such that, even if the housing, the buoyancy wings, and the power unit undergo abrupt floating movements according to the flow on the sea surface, a stable use can be guaranteed while flexibly dealing with the same.