A variable valve mechanism (12a, 12b) varies a lift amount and an opening/closing timing of an intake valve (10) of an internal combustion engine (1). During a first period in an engine start operation, wall flow is suppressed by setting the lift amount of the intake valve (10) to be smaller so as to increase the intake air velocity. During a second period, which starts after the first period ends, the atomization of injected fuel is enhanced by increasing a valve overlap amount between the intake valve (10) and an exhaust valve (11) so as to promote blow-back of the combustion gas to an intake port (4). By sequentially applying a process for increasing the intake air velocity and a process for increasing the valve overlap amount, the emission of unburned hydrocarbon during the engine start is effectively reduced.