Valve operating control system in engine

A pair of valve-timing controlling oil passages (P18a,P18b) leading to a second valve-operating characteristic changing mechanism are defined in a lower camshaft holder (25), and a valve lift controlling oil passage (P11) leading to a first valve-operating characteristic changing mechanism is defined between the pair of valve-timing controlling oil passages in a mating surface of the lower camshaft holder with a cylinder head (Fig. 14). In an area where the width of the valve lift controlling oil passage (P11) is reduced in order to avoid the interference with the valve-timing controlling oil passages (P18a,P18b), the depth of the valve lift controlling oil passage (P11) is larger than that in the other positions, whereby a sectional area of the oil passage (P11) is secured. Thus, the oil passages leading to the first and second mechanisms can be formed compact in a camshaft support member.