A failure diagnostic apparatus in an engine intake system is provided comprising a throttle valve arranged in an intake path of the engine, a detecting unit detecting an opening of the throttle valve, a detecting unit detecting an air amount passing through the intake pipe, a detecting unit detecting an output torque of the engine, an estimating unit estimating an air amount passing through the throttle valve on the basis of the opening of the throttle valve, an estimating unit estimating an air amount on the basis of the output torque, a correction unit adding time delay correction to the output data of the air amounts, and a failure judging circuitry comparing the three air amount provided by the respective unit, thereby making a judgement of a failure on any one of the detecting units.