A fuel vapor purge system includes a fuel tank, a canister, and an evaporation route, and purges fuel vapor collected in the canister into an intake passage of an internal combustion engine. A failure diagnosis apparatus of the fuel vapor purge system includes a controller having a pressure difference applying unit for applying a pressure difference between the evaporation route and outside air. The controller also detects a level of an influence of the fuel vapor generated in the fuel tank on failure determination of the evaporation route, and detects both a pressure state obtained when the pressure difference applying unit communicates with a reference orifice communicating with the outside air and a pressure state obtained when the pressure difference applying unit communicates with the evaporation route. The failure diagnosis apparatus determines whether there is a failure in the evaporation route in view of the level of the influence by comparing the detected pressure states with each other.