A cooling system for a power transmission unit that can cool a cooling site effectively is provided. The cooling system comprises: an oil pump; an oil cooler that cools the oil delivered from the oil pump; a first oil pipe for delivering the oil from the oil pump to the oil cooler; a second oil pipe for delivering the oil cooled by the oil cooler to a heat generating element. A housing chamber includes a high-temperature section where the heat generating element is arranged, and a low-temperature section. The second oil pipe is arranged in the housing chamber while passing through the low-temperature section, and an opening is formed on the second oil pipe in such a manner as to open toward the heat generating element.