A controller is applied to an internal combustion engine in which an electrically heated catalyst that is heated when supplied with electric power is installed in an exhaust passage. The controller is configured to perform a preheating process of warming up a first exhaust catalyst by supplying electric power to the electrically heated catalyst through control over a power supply before a start of the internal combustion engine. The controller is configured to, when an insulation resistance of the electrically heated catalyst at a start of the preheating process is lower than a threshold, perform the preheating process while decreasing a voltage supplied to the electrically heated catalyst.