An evaporated fuel treatment apparatus includes a control valve having a dead zone range in which an evaporated fuel is blocked even if the opening angle is increased from a close position in opening direction and a communicating range in which the evaporated fuel is allowed to flow therethrough when opening angle is increased from the dead zone range. A control unit determines whether the opening angle is switched between the dead zone range and the communicating range. The switching is determined on the basis of a pressure inside the fuel tank detected with a pressure sensor and an air-fuel ratio detected by an air-fuel ratio sensor. A control valve installed in a communication path between a fuel tank and a canister is prevented from being seized.