A malfunction detection apparatus for detecting a malfunction in an evaporated fuel purge system in which fuel vapor from a fuel tank is adsorbed in an adsorbent in a canister and the adsorbed fuel vapor in the adsorbent is purged into an intake passage of an engine. The apparatus includes a detection part for detecting a concentration of fuel in the fuel vapor purged into the intake passage so that a change in the detected fuel concentration from a time when a purge cutting is performed to a time when a purging is performed after the purge cutting has been performed is detected, and a discrimination part for determining whether there is a malfunction in the system on the basis of the change in the detected fuel concentration by the detection part. The apparatus also includes a fuel vapor detection part for detecting a condition of fuel vapor in the fuel tank, and a purge cut time varying part for varying a purge cut time period for which the purge cutting is continuously performed, the purge cut time being varied by the purge cut time varying part in response to the detected fuel vapor condition by the fuel vapor detection part.