A heat-pipe electric power generating device includes a heat pipe having an evaporating end and a condensing end, a non-magnetic shell connected to the condensing end, a generator stator coil disposed at the outer of the non-magnetic shell, a turbine disposed in the heat pipe, a driving axle connected to the turbine and extended into the non-magnetic shell, and a magnetic element disposed at the driving axle and located in the non-magnetic shell. A vapor flow flowing to the condensing end is generated at the evaporating end. The vapor flow drives the turbine to move the magnetic element, such that the generator stator coil generates an induced current. In addition, a hydrogen/oxygen gas generating apparatus and an internal combustion engine system having the heat-pipe electric power generating device are also provided.