PROBLEM TO BE SOLVED: To provide an oil supply device for an internal combustion engine that is advantageous for reducing the size of a bearing axially supporting a camshaft, and furthermore the whole length of an engine.SOLUTION: The oil supply device for an internal combustion engine includes: a camshaft 20 coupled at one end thereof to a variable valve timing mechanism 30; an oil passage 44 in the camshaft, which extends within the camshaft in the shaft direction for supplying an oil for driving to the variable valve timing mechanism; a valve system oil passage 15 for supplying an oil to an oil pressure lash adjuster; and an oil connection passage 45 for connecting the oil passage in the camshaft and the valve system oil passage, thus allowing the oil to be supplied to the oil passage in the camshaft after the oil flows through the valve system oil passage.