The internal combustion engine has a valve driving mechanism (VM) capable of changing the valve-opening characteristic of at least one of an intake valve (Vi) and an exhaust valve (Ve), an in-cylinder pressure sensor for detecting the in-cylinder pressure in a combustion chamber and ECU. ECU calculates the variation amount of the in-cylinder pressure caused by the valve-overlap of the intake valve (Vi) and the exhaust valve (Ve), and based on this variation amount of the in-cylinder pressure and the in-cylinder pressure detected at a predetermined timing in the compression stroke, calculates an amount of air sucked in the combustion chamber, as well as, based on this calculated intake air amount, determines the ignition timing. The amount of air sucked in the combustion chamber is accurately and costlessly calculated, and the ignition timing is optimally determined by using the calculated air amount.