In a specified operating area including a low engine-speed and low engine-load area, an excess air ratio λ is set at a specified ratio which is two or greater and three or smaller and an ignition timing θig of a spark plug is set at a timing of MBT to provide the maximum torque as a normal combustion control (S4). In case a catalyst temperature Tc is lower than a predetermined first temperature T1 in this area, a control of retarding the ignition timing from the MBT timing and/or a control of decreasing the excess air ratio λ to a ration below the above-described ratio are executed (S6, S8, S9). Accordingly, the temperature of the catalyst can be prevented simply and effectively from decreasing excessively, maintaining combustion conditions to provide a properly high thermal efficiency.