A diagnostic apparatus for an internal combustion engine, comprising an adjusting device for adjusting a burning condition, a correction device for estimating or detecting an input or an output to and from the internal combustion engine so as to correct at least one of a throttle opening degree, a fuel injection pulse width, a fuel injection timing, an ignition timing and an exhaust gas recirculation quantity, a recording device for recoding a controlled correction value from the correction device at every predetermined time, and a fuel consumption evaluation means for evaluating a fuel consumption of the engine, being based upon the recorded data or log data as to the recorded data, wherein a deterioration of fuel consumption caused by an abnormality of the engine can been soon detected with no affection by an operating condition of the engine and unevenness of component parts.