A valve operating mechanism for an internal combustion engine having a plurality of exhaust valves (1,2) per cylinder comprises means for driving the exhaust valves at such a valve timing that makes opening of the exhaust valves cause blowdown pressures with a phase difference (0), i.e., at different timings (E1,E2) relative to a crank angle. In one aspect of this invention, the exhaust valves open at the different times and close at the same time.