Evaporated fuel treatment apparatus and method of detecting failure in control valve

An evaporated fuel treatment apparatus comprising:na canister configured to adsorb an evaporated fuel generated in a fuel tank; anda control valve, installed at a vapor path communicating with a fuel tank and a canister, configured to allow an evaporated fuel to flow therethrough, have a dead-zone range in an opening angle of a valve element where a flow of the evaporated fuel is blocked even when the opening angle of the control valve is increased in an open direction from the opening angle of zero that is a closed position of the control valve, and allow the evaporated fuel to flow therethrough when the opening angle of the control valve exceeds the dead-zone range; anda control unit configured to perform opening control on the control valve so as to cause the evaporated fuel to flow through the control valve,wherein the control unit increases the opening angle of the control valve from substantially zero to a predetermined opening angle in the dead-zone range and stands by prior to the opening control, and increases the opening angle of the control valve from the predetermined opening angle at the time of opening control or changes an opening speed of the control valve between a first range of the opening angle from substantially zero to the predetermined opening angle in the dead-zone range and a second range of the opening angle exceeding the predetermined opening angle.