A control apparatus for an internal-combustion engine includes a diagnosis device. The diagnosis device includes a determination section and an exhaust flow rate calculator configured to calculate an exhaust flow rate that reflects a volume flow amount of exhaust gas. A filter is configured to filter a signal representing an air-fuel ratio so that a 0.5th-order frequency component of engine speed is extracted from the signal. An integrator is configured to accumulate a filtered signal for a predetermined period of time to compute an integration value. A corrector is configured to correct the integration value to compute a corrected integration value each time the integration value is computed by the integrator. The determination section is configured to determine that cylinder-to-cylinder air-fuel ratio imbalance occurs in cylinders if the corrected integration value is greater than a predetermined threshold value after the predetermined period of time has elapsed.