An engine fuel diagnosis device is disclosed that diagnoses whether an unexpected fuel is being used in an internal combustion engine. The fuel diagnosis device includes an inclination detecting section, a first estimating section, an acceleration detecting section, and a diagnosing section. The first estimating section computes a first estimated acceleration, which is a vehicle acceleration obtained when fuel injection is executed in the internal combustion engine while the vehicle is traveling uphill or downhill, based on an engine power, which is estimated based on the engine operation state on the assumption that a previously expected fuel is being used, and the inclination detected by the inclination detecting section. The diagnosing section computes a first deviation degree indicating the degree of deviation between the first estimated acceleration and the actual acceleration detected when the first estimated acceleration is computed. The diagnosing section diagnoses the usage state of the unexpected fuel based on the first deviation degree.