An internal combustion engine which includes a valve deactivation mechanism for reducing an output power shock upon changing of the cylinder number. In an internal combustion engine which includes a valve deactivation mechanism driven by a slide pin which is driven by hydraulic pressure, response delay time after a signal is sent to oil control valves until an intake valve and an exhaust valve are activated or deactivated is used to form a control map in response to control parameters, and the valve deactivation mechanism is controlled based on the control map.