PURPOSE: To make an engine compact and to make a valve system endurable against high speed rotation by rotating two pieces each of roller valves on both sides installed in a cylinder head in the opposite directions respectively.

CONSTITUTION: When a roller valve timing gear 1 is rotated by rotation of a crankshaft, a directly connected roller valve 2 is rotated. Then, by a drive gear 3 installed on the roller valve 2, a driven gear 5 installed on a roller valve 4 directly is rotated and the roller valve 4 is rotated. Then, the roller valve on one side opens and closes. Other roller valves 6, 7 do the same. A shim for vompression leakage prevention is used on a part of a housing of the roller valves. Consequently, it is possible to make an engine main body compact, make it a valve system endurable against high speed rotation and to minimize the number of parts of the engine.