The present embodiment relates to a DC-DC converter including: a housing; a plurality of electronic components disposed inside the housing; and a flow path disposed on a lower plate of the housing. The flow path includes an expanding portion. The horizontal width of the expanding portion is greater than the horizontal width of a flow path on the front end of the expanding portion, and the vertical width of the expanding portion is less than the vertical width of the flow path on the front end of the expanding portion. The differential between the part wherein the surface area of the vertical cross section of the flow path is the biggest and the part wherein the surface area of the vertical cross section of the flow path is the smallest is 10% or less.