By employing command value signals for controlling the opening timing of an exhaust valve (11) or an intake valve (10) and means (20) for measuring flow rates of an intake air of a flow direction which flows through an intake manifold (19) of an internal combustion engine (1), an air flow rate corresponding to the command value signal is measured. A valve mechanism (40, 41) includes the intake valve (10) and the exhaust valve (11) for each cylinder (1a) of the internal combustion engine (1) and is controlled by a valve control means (30) for controlling the valve mechanism (40, 41). A failure diagnosis of an operating status of the valve mechanism (40, 41) can be performed.