An engine control system designed to allow homogeneous-charge compression ignition combustion to be performed in an HCCI region defined as an engine operating region including at least a partial-load range of an engine, comprising: a variable lift mechanism adapted to variably set a lift amount of an intake valve; a switching mechanism adapted to be capable of switching between allowing an exhaust valve to be opened both during an exhaust stroke and during an intake stroke, and allowing the exhaust valve to be opened only during the exhaust stroke; and a valve controller adapted to drive the variable lift mechanism and the switching mechanism to control respective opening/closing operations of the intake valve and the exhaust valve, wherein the HCCI region is divided into a plurality of load zones (R1-R4) including a low load zone, a medium load zone (R2, R3) in which the engine load is higher than in the low load zone, and a high load zone in which the engine load is higher than in the medium load zone, characterized in that the valve controller is operable: in the low load zone of the HCCI region, to set the lift amount of the intake valve to a first predetermined value less than a lift amount of the exhaust valve opened during the intake stroke, and allow the exhaust valve to start being opened during the intake stroke at a given timing later than an opening timing of the intake valve; in the medium load zone (R2, R3) of the HCCI region, to gradually increase the lift amount of the intake valve up to a second predetermined value equal to or greater than the lift amount of the exhaust valve opened during the intake stroke, along with an increase in the engine load, and set the lift amount of the exhaust valve opened during the intake stroke to a value equal to the lift amount thereof in the low load zone; and in the high load zone of the HCCI region, to inhibit the exhaust valve from being opened during the intake stroke so as to set the number of the exhaust valves to be opened during the intake stroke, to zero. a variable lift mechanism adapted to variably set a lift amount of an intake valve; a switching mechanism adapted to be capable of switching between allowing an exhaust valve to be opened both during an exhaust stroke and during an intake stroke, and allowing the exhaust valve to be opened only during the exhaust stroke; and a valve controller adapted to drive the variable lift mechanism and the switching mechanism to control respective opening/closing operations of the intake valve and the exhaust valve, wherein the HCCI region is divided into a plurality of load zones (R1-R4) including a low load zone, a medium load zone (R2, R3) in which the engine load is higher than in the low load zone, and a high load zone in which the engine load is higher than in the medium load zone, in the low load zone of the HCCI region, to set the lift amount of the intake valve to a first predetermined value less than a lift amount of the exhaust valve opened during the intake stroke, and allow the exhaust valve to start being opened during the intake stroke at a given timing later than an opening timing of the intake valve; in the medium load zone (R2, R3) of the HCCI region, to gradually increase the lift amount of the intake valve up to a second predetermined value equal to or greater than the lift amount of the exhaust valve opened during the intake stroke, along with an increase in the engine load, and set the lift amount of the exhaust valve opened during the intake stroke to a value equal to the lift amount thereof in the low load zone; and in the high load zone of the HCCI region, to inhibit the exhaust valve from being opened during the intake stroke so as to set the number of the exhaust valves to be opened during the intake stroke, to zero.