An oil supply structure for a variable valve timing mechanism that adjusts the valve timing of an exhaust valve of an engine. A exhaust side camshaft having a journal is provide for actuating the exhaust valve. A cylinder head and a bearing cap rotatably support the camshaft at its journal. A pulley is mounted on the distal end of the camshaft, and the pulley is rotatable relative to the camshaft. A belt connects the pulley to the crankshaft to transmit power from the engine to the pulley. The belt applies a force to the pulley and the camshaft. A ring gear is provided for changing the relative rotational relationship between the camshaft and the pulley. First and second chambers are defined for applying a hydraulic fluid pressure to the ring gear. First and second passages are defined in the camshaft, and the first and the second passages are connected to the first and second chambers, respectively. First and second conduits are formed in the cylinder head, and the first and second conduits are connected to the first and second passages, respectively. An oil pressure is supplied to the second chamber via the second conduit and the second passage to advance the valve timing of the exhaust valve with respect to the crankshaft. The second conduit and the second passage are connected at a position closer to the pulley than where the first conduit and the first passage are connected.