A valve operating system for an automotive internal combustion engine. The valve operating system is comprised of a plurality of bearing sections formed in a cylinder head of the engine and adapted to rotatably support a camshaft formed with a plurality of bearing journal sections. Each bearing section has a generally cylindrical bearing surface defining a bore in which each camshaft bearing journal section is disposed. The peripheral surface of the camshaft bearing journal section is slidably contactable with the bearing section bearing surface. In a cross-section to which the axis of the bore is perpendicular, the bearing surface has a first diametrical distance in which reaction of a valve spring for an intake or exhaust valve is applied, and a second diametrical distance perpendicular to the above-mentioned direction. The first diametrical distance is smaller than the second diametrical distance, thereby forming larger an oil clearance correponding to the first diametrical distance than an oil clearance corresponding to the second diametrical distance.