Herein disclosed is a lubricant heating system for an internal combustion engine equipped with an exhaust pipe which is connected with a heat exchanger. This heat exchanger is constructed to include a communication passage, which communicates with the exhaust pipe so as to be heated by the engine exhaust gas, and a radiating unit which is disposed adjacent to the communication passage and contacts with the lubricant of the internal combustion engine thereby to heat the lubricant with the heat in the exhaust gas transferred from the communication passage. The heat exchanger thus constructed is adapted to promptly heat the lubricant, which is mixed with a fuel, by the use of the exhaust heat of the internal combustion engine, thus promoting the warm-up of the engine. Moreover, impurities such as the mixed fuel or water are evaporated and separated from the lubricant and are promptly discharged out as a blow-by gas.