Disclosed is an engine control system which is designed to allow homogeneous-charge compression ignition combustion to be performed in an HCCI region (R) defined as an engine operating region including at least a partial-load range of an engine. In a low load zone (R1) of the HCCI region (R), a lift amount of an intake valve (11) is set to a first predetermined value, and an exhaust valve (12) is allowed to start being opened during an intake stroke at a given timing later than an opening timing of the intake valve (11). Further, in the medium load zone of the HCCI region, the lift amount of the intake valve (11) is gradually increased up to a second predetermined value greater than the first predetermined value, along with an increase in an engine load. The engine control system is capable of adequately controlling an amount of burned gas to be introduced into a cylinder, depending on the engine load to allow the HCCI combustion to be adequately performed in a wider engine load range.