A malfunction detecting device for an internal combustion engine is mounted on a vehicle and includes an engine and a control unit. The engine is a multi-cylinder engine and is a bi fuel-enabled engine which is capable of switching plural kinds of fuels including a gas fuel and which can use bifuel. The control unit detects, in case of detecting a malfunction with respect to an imbalance of an air fuel ratio among the cylinders at a time of an operation based on the gas fuel, the malfunction based on variation of pressure pulsations of the gas fuel due to a fuel injection.