A failure determination system of exhaust gas purification apparatus comprising: a NOx selective catalytic reduction catalyst that is disposed in an exhaust passage of an internal combustion engine and uses ammonia as a reducing agent; a reducing agent supply unit configured to supply ammonia or a precursor of ammonia into exhaust gas flowing into the NOx selective catalytic reduction catalyst at a location upstream from the NOx selective catalytic reduction catalyst; and a NOx sensor that is disposed downstream from the NOx selective catalytic reduction catalyst and is configured to detect NOx in exhaust gas flowing out of the NOx selective catalytic reduction catalyst; and a failure determination unit configured to perform failure determination of the exhaust gas purification apparatus based on a detected value of the NOx sensor; characterised by comprising: a calculation unit configured to calculate an adsorption amount difference at assumed failure, the adsorption amount difference at assumed failure being a difference between an ammonia equilibrium adsorption amount in the NOx selective catalytic reduction catalyst in an equilibrium state of ammonia adsorption, assuming that the exhaust gas purification apparatus is in a predefined failure state, and an actual ammonia adsorption amount in the NOx selective catalytic reduction catalyst; and a failure determination restriction unit configured to restrict a use of the detected value of the NOx sensor in the failure determination by the failure determination unit or to inhibit the failure determination itself by the failure determination unit, when the adsorption amount difference at assumed failure calculated by the calculation unit exceeds a predetermined value. a NOx selective catalytic reduction catalyst that is disposed in an exhaust passage of an internal combustion engine and uses ammonia as a reducing agent; a reducing agent supply unit configured to supply ammonia or a precursor of ammonia into exhaust gas flowing into the NOx selective catalytic reduction catalyst at a location upstream from the NOx selective catalytic reduction catalyst; and a NOx sensor that is disposed downstream from the NOx selective catalytic reduction catalyst and is configured to detect NOx in exhaust gas flowing out of the NOx selective catalytic reduction catalyst; and a failure determination unit configured to perform failure determination of the exhaust gas purification apparatus based on a detected value of the NOx sensor; characterised by comprising: a calculation unit configured to calculate an adsorption amount difference at assumed failure, the adsorption amount difference at assumed failure being a difference between an ammonia equilibrium adsorption amount in the NOx selective catalytic reduction catalyst in an equilibrium state of ammonia adsorption, assuming that the exhaust gas purification apparatus is in a predefined failure state, and an actual ammonia adsorption amount in the NOx selective catalytic reduction catalyst; and a failure determination restriction unit configured to restrict a use of the detected value of the NOx sensor in the failure determination by the failure determination unit or to inhibit the failure determination itself by the failure determination unit, when the adsorption amount difference at assumed failure calculated by the calculation unit exceeds a predetermined value. a calculation unit configured to calculate an adsorption amount difference at assumed failure, the adsorption amount difference at assumed failure being a difference between an ammonia equilibrium adsorption amount in the NOx selective catalytic reduction catalyst in an equilibrium state of ammonia adsorption, assuming that the exhaust gas purification apparatus is in a predefined failure state, and an actual ammonia adsorption amount in the NOx selective catalytic reduction catalyst; and a failure determination restriction unit configured to restrict a use of the detected value of the NOx sensor in the failure determination by the failure determination unit or to inhibit the failure determination itself by the failure determination unit, when the adsorption amount difference at assumed failure calculated by the calculation unit exceeds a predetermined value.