A valve system of an internal combustion engine mounted on a vehicle comprises a valve characteristic varying mechanism for controlling valve operation characteristics of an engine valve, and an electric motor of the valve characteristic varying mechanism is disposed in the exterior of a valve chamber defined by the cylinder head. The cylinder head is provided with a duct, for guiding a running airflow therethrough, between a combustion chamber and the valve chamber. The electric motor is laid out at a position which is adjacent to the valve chamber in the radial direction with respect to the cylinder axis and at which the running airflow having flowed in via an inlet portion and having passed through the duct collides on the electric motor.