An electric power generating apparatus is mounted on a vehicle which has a propulsive electric motor powered by a battery, and has an electric generator and an internal combustion engine for actuating the electric generator to charge the battery depending on a charged/discharged state of the battery. A first operation controller operates the internal combustion engine to enable the electric generator to charge the battery if the amount of electric energy stored in the battery is smaller than a predetermined value or the time-dependent rate of increase of an amount of electric energy discharged from the battery is greater than a predetermined level. A second operation controller operates the internal combustion engine to purge a canister if the charged/discharged state of the battery fails to satisfy a condition for the first operation controller to operate the internal combustion engine and if the amount of a purged gas adsorbed in the canister is greater than a predetermined amount. The first and second operation controllers operate the internal combustion engine in modes established respectively for the first and second operation controllers such that a load imposed on the internal combustion engine when the internal combustion engine is operated by at least the second operation controller is smaller than a load imposed on the internal combustion engine when the internal combustion engine is operated by the first operation controller.