To reduce size, weight and cost of a slider gear of an intermediary phase difference variable means, reduce drive force necessary for moving the slider gear and improve assembly workability.

The intermediary phase difference variable means of a variable valve system is provided with a helical spline 22 provided on an inner circumference surface of an input part 22, helical splines 24b, 26b provided on inner circumference surfaces of rocking cams 24, 26, the slider gear 28 including an input helical spline 28a and output helical splines 28c, 28e on an outer circumference surface of a shaft body and relatively rocking the input part 22 and the rocking cams 24, 26 according to move to an axial direction by meshing thereof, and a displacement adjustment means thereof. The slider gear 28 includes the input spline 28a and the output splines 28c, 28e only in a part of a center angle range of the outer circumference surface of the shaft bodies respectively.