An apparatus for converting vibratory mechanical energy into electrical energy includes a mobile mass, a support, first and second beams, the second being piezoelectric, and a junction element. The first beam extends longitudinally between the support and the mass, each of which has a beam end embedded therein. The second beam links the support and the mobile mass. Its elongation stiffness is lower than that of the first beam. The junction element extends between the beams. A first assembly, with a first bending stiffness, comprises the first beam, the second beam, and the junction element. A second assembly consists of the first assembly minus the second beam. Its bending stiffness is less than or equal to half of that of the first assembly.