An abnormality diagnosis apparatus for an NOx catalyst of the present invention measures an index value (R) regarding an NOx clarification capacity of an NOx catalyst and determines an abnormality of the NOx catalyst based on index values (R1, R2) respectively measured at a timing (Tc1) at which the NOx catalyst has a relatively-low activation level and a timing (Tc2) at which the NOx catalyst has a relatively-high activation level. Since the index value at the timing at which the NOx catalyst has a relatively-low activation level (i.e., a timing at which the NOx catalyst originally does not have a sufficient NOx clarification capacity) is used, the abnormality diagnosis for the NOx catalyst can be carried out without intentionally deteriorating emission.