A glow power selected from a map based on a coolant temperature and an intake air temperature is applied to a glow plug provided at a combustion cylinder after an engine starts. Air-fuel mixture is heated and combusted, and then a rotation variation is calculated from a difference between a maximum rotating speed and a minimum rotating speed of the combustion cylinder. If the rotation variation is outside an allowable range with reference to an average rotation variation for four cylinders, the glow power corresponding to the combustion cylinder is corrected by a correction value set based on the rotation variation, thereby equalizing rotation variations for all cylinders.