A malfunction diagnosis apparatus diagnoses a malfunction of an exhaust gas sensor provided on a downstream side of a catalyst. A dead time determination unit determines a dead time between a first time point, at which fuel cut of the internal combustion engine is started, and a second time point, at which the fuel cut starts exerting effect on an output of the exhaust gas sensor. An output change time measurement unit measures an output change time between the first time point and a third time point, at which the output of the exhaust gas sensor changes to a leaner side than a threshold. A malfunction determination unit determines whether a malfunction is caused in the exhaust gas sensor on the basis of a response time of the exhaust gas sensor obtained by subtracting the dead time from the output change time.