The present invention enables a large leak to be accurately detected by providing an evaporated fuel treatment apparatus for an internal combustion engine. The apparatus comprises the evaporated fuel discharge prevention system including a fuel tank, a canister having an opening to the atmosphere, a passage allowing the fuel tank to communicate with the canister, a purging passage allowing the canister to communicate with the intake manifold of the engine. The apparatus also comprises a pressure sensor for detecting the pressure of the evaporated fuel discharge prevention system, and a controller coupled to the pressure sensor for judging the presence of a first leak in the evaporated fuel discharge prevention system if a change in the pressure from the pressure sensor is small. The controller further checks a change in the pressure from said pressure sensor when closing said system after placing said system under negative pressure. The controller judges the presence of a large leak if the first leak is judged and the pressure increases instantaneously upon closing said system.