A human powered electric energy generator equipped with a mechanism for converting a back and forth motion of a user's legs or arms to a unidirectional circular motion. The device includes an engaging mechanism that, according to the back and forth movement of a belt, enables a pilot gear to alternate engagement between a left and a right gear, and a transmission mechanism which transfers the linear kinetic energy from the belt to a unidirectional rotational kinetic energy of the gears. The device also includes a flywheel for storing the rotational kinetic energy of the gears and/or the transmission mechanism, and an electric generator for converting the rotational kinetic energy into electric energy.