An internal combustion engine for a small planing boat for reducing the lateral width of an oil pan in conformity to the configuration of the bottom while securing the filtering surface area of an oil strainer. In addition, a vertical width of the oil pan is maintained to be small to thereby lower the total height of the internal combustion engine. In an internal combustion engine for a small planing boat, an oil pan is provided at the bottom portion of the internal combustion engine with an oil passage for communicating with an oil pump being formed inside the oil pan. A vertical wall of the oil passage is partially cut away to form communication openings each communicating between the inside and outside of the oil passage with the oil strainers being interposed in the communication openings substantially vertically to divide the oil passage into inside and outside.