An abnormality diagnosis apparatus for an exhaust gas purification apparatus, comprising: an exhaust gas purification apparatus disposed in an exhaust passage of an internal combustion engine and having a selective catalytic reduction catalyst; an addition device (5, 50, 51) configured to add an additive constituted by ammonia or an ammonia precursor to exhaust gas flowing into said exhaust gas purification apparatus; estimating means configured to estimate a NO x inflow amount, which is an amount of NO x flowing into said exhaust gas purification apparatus, using a parameter indicating an operating condition of said internal combustion engine; first obtaining means configured to obtain an NH 3 adsorption amount, which is an amount of ammonia adsorbed to said exhaust gas purification apparatus, using said NO x inflow amount estimated by said estimating means as a parameter; control means configured to control an amount of said additive added by said addition device, using said NH 3 adsorption amount obtained by said first obtaining means as a parameter; and diagnosing means configured to determine whether or not an abnormality exists in said exhaust gas purification apparatus by calculating a physical quantity that correlates with a NO X purification ability of said exhaust gas purification apparatus, using said NO X inflow amount estimated by said estimating means as a parameter, and comparing a calculation result with a predetermined threshold, said abnormality diagnosis apparatus further comprising second obtaining means configured to obtain a minimum NH 3 adsorption amount, which is an NH 3 adsorption amount of said exhaust gas purification apparatus obtained on the supposition that said exhaust gas purification apparatus is normal and that an amount of NO X discharged from said internal combustion engine reaches a maximum under an identical operating condition to said operating condition of said internal combustion engine in which said NO X inflow amount is estimated by said estimating means, wherein said diagnosing means are configured to determine a whether or not said NO X purification ability of said exhaust gas purification apparatus has deteriorated from a normal condition by comparing said physical quantity with a first threshold when said minimum NH 3 adsorption amount equals or exceeds a predetermined amount, and determines whether or not said NO X purification ability of said exhaust gas purification apparatus has been completely lost by comparing said physical quantity with a second threshold that is smaller than said first threshold when said minimum NH 3 adsorption amount is smaller than said predetermined amount. an exhaust gas purification apparatus disposed in an exhaust passage of an internal combustion engine and having a selective catalytic reduction catalyst; an addition device (5, 50, 51) configured to add an additive constituted by ammonia or an ammonia precursor to exhaust gas flowing into said exhaust gas purification apparatus; estimating means configured to estimate a NO x inflow amount, which is an amount of NO x flowing into said exhaust gas purification apparatus, using a parameter indicating an operating condition of said internal combustion engine; first obtaining means configured to obtain an NH 3 adsorption amount, which is an amount of ammonia adsorbed to said exhaust gas purification apparatus, using said NO x inflow amount estimated by said estimating means as a parameter; control means configured to control an amount of said additive added by said addition device, using said NH 3 adsorption amount obtained by said first obtaining means as a parameter; and diagnosing means configured to determine whether or not an abnormality exists in said exhaust gas purification apparatus by calculating a physical quantity that correlates with a NO X purification ability of said exhaust gas purification apparatus, using said NO X inflow amount estimated by said estimating means as a parameter, and comparing a calculation result with a predetermined threshold, said abnormality diagnosis apparatus further comprising second obtaining means configured to obtain a minimum NH 3 adsorption amount, which is an NH 3 adsorption amount of said exhaust gas purification apparatus obtained on the supposition that said exhaust gas purification apparatus is normal and that an amount of NO X discharged from said internal combustion engine reaches a maximum under an identical operating condition to said operating condition of said internal combustion engine in which said NO X inflow amount is estimated by said estimating means, wherein said diagnosing means are configured to determine a whether or not said NO X purification ability of said exhaust gas purification apparatus has deteriorated from a normal condition by comparing said physical quantity with a first threshold when said minimum NH 3 adsorption amount equals or exceeds a predetermined amount, and determines whether or not said NO X purification ability of said exhaust gas purification apparatus has been completely lost by comparing said physical quantity with a second threshold that is smaller than said first threshold when said minimum NH 3 adsorption amount is smaller than said predetermined amount.