PURPOSE: To provide an oxygen sensor abnormality detecting device capable of correctly detecting the abnormality of a downstream oxygen sensor.

CONSTITUTION: A first by-pass passage 9 is branched from an exhaust pipe 3 so as to let exhaust gas flow while making a detour around a catalytic converter 5. When the exhaust gas is let flow in the direction of the first by-pass passage 9 by a first by-pass valve 11 and the flow of the exhaust gas in the direction of a muffler from the first by-pass passage 9 is shut off by a second by-pass valve 19, the exhaust gas upstream of the catalytic converter 5 flows directly to a downstream oxygen sensor 17 and exhausted through a second by-pass passage 15 after flowing reversely in the catalytic converter 5. The output of the downstream oxygen sensor 17 at this time is compared with the output of an upstream oxygen sensor 7 positioned upstream of the first by-pass valve 11 to detect the abnormality of the downstream oxygen sensor 17.