A leakage diagnosis apparatus is applied to an evaporated-gas purge system including a negative-pressure sensor and a pressure detecting portion. The negative-pressure sensor introduces a negative pressure into an evaporation system. The pressure detecting portion detects the pressure in the evaporation system. The leakage diagnosis apparatus includes a leakage-diagnosis portion and a determining portion. The leakage-diagnosis portion executes a leakage diagnosis to determine whether a leakage of the evaporation system is generated. The determining portion determines whether an evaporated-gas amount in the evaporation system is in an excessive state while the internal combustion engine is stopped. The leakage-diagnosis portion prohibits or terminates the leakage diagnosis, when the determining portion determines that the evaporated-gas amount is in the excessive state.