In a lubrication system of an internal combustion engine provided with a generator that generates electricity by making use of the power generated by the engine, and a heating mechanism that heats lubricating oil of the engine by making use of the energy generated when the generator carries out a power generation operation, a comparison is made between an amount of decrease of an engine load in cases where the heating mechanism heats the lubricating oil and an amount of increase of the engine load in cases where the generator carries out the power generation operation for the purpose of the operation of the heating mechanism, and permits the power generation operation of the generator in order to supply energy to the heating mechanism in cases where the amount of decrease of the engine load exceeds the amount of increase of the engine load.