The present invention provides an abnormality diagnosis apparatus for an exhaust gas purification apparatus having an SCR catalyst, with which a misdiagnosis of an abnormality in the SCR catalyst even though the SCR catalyst is normal can be suppressed. In the present invention, a NOx purification ratio is determined using an estimated NOx inflow amount as a parameter, and an abnormality in the SCR catalyst is diagnosed on the basis of the NOx purification ratio. According to the present invention, a minimum NH3 adsorption amount is determined as an NH3 adsorption amount of the SCR catalyst in a condition where an actual NOx inflow amount reaches a maximum. When the minimum NH3 adsorption amount equals or exceeds a predetermined amount, a determination is made as to whether or not a NOx purification ability of the SCR catalyst has deteriorated from a normal condition by comparing the NOx purification ratio with a first threshold. When the minimum NH3 adsorption amount is smaller than the predetermined amount, a determination is made as to whether or not the NOx purification ability of the SCR catalyst has been completely lost by comparing the NOx purification ratio with a second threshold that is smaller than the first threshold.