To provide an abnormality diagnosis device for an oxygen concentration sensor capable of restraining erroneous determination caused by oxygen storage action of a catalyst when diagnosing abnormality of the oxygen concentration sensor disposed in the downstream side of the catalyst.

A first output voltage V1 which is the value just after fuel cut is started and a second output voltage V2 which is the value just after fresh air reaches an upstream side oxygen concentration sensor disposed on the upstream side of the catalyst are stored, as to an output value of the downstream side oxygen concentration sensor provided on the downstream side of the catalyst. When an output difference V which is the difference between the first output voltage V1 and the second output voltage V2 is equal to or more than a determination value A (S170: YES), or the second output voltage V2 is equal to or lower than a determination value B (S190: YES) even when the output difference V is lower than the determination value A (S170: NO), measurement of response time RT of the downstream side oxygen concentration sensor is stopped (S180), and abnormality diagnosis of the downstream side oxygen concentration sensor is forbidden (S185).