A control device is applied to an internal combustion engine capable of using CNG and gasoline by switching therebetween, and the CNG and the gasoline are associated, as fuel for use, with one and the other of two driving areas respectively, the driving areas being different from each other. The control device exceptionally switches the fuel for use from gasoline to CNG and implements an air-fuel ratio imbalance inspection (S6, S7), when the air-fuel ratio imbalance inspection is required for a CNG mode using CNG (S1), the engine is driving in a driving area (AR2) correlated to gasoline as fuel for use (S2), and gasoline mode using gasoline as fuel for use is ongoing (S3).