A diagnostic device of an internal combustion engine diagnoses a condition of a differential pressure sensor. The sensor in a normal condition detects a differential pressure of an exhaust gas between upstream and downstream sides of a diesel particulate filter disposed in an exhaust system of the engine. The sensor in a fault condition detects no differential pressure of the exhaust gas. The device has a block obtaining a first signal from the sensor when the engine is operated in a low flow rate region of the exhaust gas and obtaining a second signal from the sensor when the engine is operated in a high flow rate region of the exhaust gas. A diagnostic block of the device judges from a difference between levels of the first and second signals whether the sensor is in the normal or fault condition.