Compression ratio controlling apparatus and method for spark-ignited internal combustion engine

In a compression ratio controlling apparatus and method for a spark-ignited internal combustion engine, the variable compression ratio mechanism is controlled by a compression controlling section on the basis of a detected engine speed and engine load in such a manner that the compression ratio is varied toward a target high compression ratio when the engine load falls in a predetermined low load region and toward a target low compression ratio when the engine load falls in a predetermined high load region and a predetermined delay is provided in a variation in the compression ratio toward one of the target high and low compression ratios in accordance with at least one of an engine driving history immediately before a transient state of a change in the engine load occurs and a wall temperature of a combustion chamber of the engine immediately before the transient state thereof occurs.