A control method for an internal combustion engine which is arranged to vary an engine compression ratio, and which includes a control mechanism arranged to control the engine compression ratio, wherein the control mechanism comprises a rotation shaft, a control shaft, a link mechanism mechanically connecting the rotation shaft and the control shaft, and a driving source arranged to drive and rotate the rotation shaft, the driving force of the driving source being transmitted through the rotation shaft and the link mechanism to the control shaft so as to rotate the control shaft, and thereby to vary the engine compression ratio, and wherein the link mechanism is arranged to vary a reduction ratio which is a ratio between a rotation amount of the rotation shaft and a rotation amount of the control shaft, in accordance with the rotation position of the control shaft, the control method comprising: converting an operation amount of the driving source in accordance with the reduction ratio according to the engine compression ratio; and transmitting the converted operation amount to the control mechanism; the control method further comprising: setting the reduction ratio at a predetermined middle compression ratio to be smaller than the reduction ratios at a high compression rate higher than the middle compression ratio, and at a low compression rate lower than the middle compression rate, the control method being characterized by controlling the cylinder pressure based on a basic ignition timing when the reduction ratio is larger than the reduction ratio at the predetermined middle compression ratio, and controlling the cylinder pressure based on an allowable ignition timing retarded from the basic ignition timing at the predetermined middle compression ratio at which the reduction ratio is small. converting an operation amount of the driving source in accordance with the reduction ratio according to the engine compression ratio; and transmitting the converted operation amount to the control mechanism; the control method further comprising: setting the reduction ratio at a predetermined middle compression ratio to be smaller than the reduction ratios at a high compression rate higher than the middle compression ratio, and at a low compression rate lower than the middle compression rate, the control method being characterized by controlling the cylinder pressure based on a basic ignition timing when the reduction ratio is larger than the reduction ratio at the predetermined middle compression ratio, and controlling the cylinder pressure based on an allowable ignition timing retarded from the basic ignition timing at the predetermined middle compression ratio at which the reduction ratio is small. setting the reduction ratio at a predetermined middle compression ratio to be smaller than the reduction ratios at a high compression rate higher than the middle compression ratio, and at a low compression rate lower than the middle compression rate, the control method being characterized by controlling the cylinder pressure based on a basic ignition timing when the reduction ratio is larger than the reduction ratio at the predetermined middle compression ratio, and controlling the cylinder pressure based on an allowable ignition timing retarded from the basic ignition timing at the predetermined middle compression ratio at which the reduction ratio is small.