Method and apparatus for determining state of reformer

An internal combustion engine (1) includes a reformer (20) for reforming a fuel air mixture to produce a reformed gas, a crank angle sensor (19) for detecting a rotational speed of the engine (1), and an ECU (50). The ECU (50) obtains a difference of an actual rotational speed from a target rotational speed of the engine (1) during an idling thereof based on a signal from the crank angle sensor (19). When the difference thus obtained exceeds a predetermined threshold value, the ECU (50) determines that a reforming catalyst of the reformer (20) has deteriorated or there is any abnormality in the reformer (20).