A control device for a multi-cylinder internal combustion engine that is equipped with a variable compression ratio mechanism includes an air-fuel ratio sensor, and a controller that determines whether or not actual mechanical compression ratios in cylinders of the internal combustion engine are uniform. The controller controls the variable compression ratio mechanism by decreasing a target mechanical compression ratio from a current first target mechanical compression ratio to a second target mechanical compression ratio without changing the amount of intake air and a fuel injection amount, and determines that the actual mechanical compression ratios in the cylinders are not uniform when the target mechanical compression ratio is set at the first target mechanical compression ratio if the differences in the output air-fuel ratios from the air-fuel ratio sensor for exhaust gases from the cylinders before and after the control of the variable compression ratio mechanism are not uniform.