A heat-pipe electric power generating device including a fan disposed between an evaporating end and a condensing end of a heat-pipe is provided. A magnetic substance is disposed on the fan to form a magnetic field. A stator coil of a generator is disposed at the outer of the heat-pipe, which is corresponding to the position of the fan. An induced current is generated by the stator coil of the generator when the magnetic substance spins. Since the heat-pipe is made of copper, and the magnetic field is not shielded by copper, a current is induced when a relative motion between the magnetic substance on the fan and the stator coil of the generator at the outer of the heat-pipe is generated. Further, the heat-pipe electric power generating device can be applied on a hydrogen/oxygen gas generating apparatus and an internal combustion engine system of a motor vehicle.