A fuel vapor treatment apparatus is provided with an air-fuel-ratio obtaining portion and a clogging determining portion. The air-fuel-ratio obtaining portion obtains the air-fuel ratio. The clogging determining portion determines whether a purge line is clogged based on a maximum variation amount of air-fuel ratio. In a case where no clogging is occurred in the purge line, the air-fuel ratio significantly fluctuates temporarily. In a case where at least a part of the purge line is fully clogged, even when a purge valve receives a valve opening command to be opened, fuel vapor cannot be introduced into the engine, so that the air-fuel ratio is not varied. In view of an air-fuel ratio variation, it is determined whether the purge line is clogged.