According to a first aspect of the present invention, there is provided an apparatus for detecting variation abnormality in an air-fuel ratio between cylinders comprising a wide-range air-fuel ratio sensor (42) and an O 2 sensor (56) provided in an exhaust passage upstream of an exhaust gas purifying apparatus (40) arranged in the exhaust passage (38) for an internal combustion engine (10) having a plurality of cylinders, air-fuel ratio controlling means for performing air-fuel ratio control for a predetermined period in such a manner as to make an exhaust air-fuel ratio be equal to a stoichiometric air-fuel ratio based upon output from the wide-range air-fuel ratio sensor (42), and abnormality detecting means for detecting variation abnormality in an air-fuel ratio between cylinders based upon output from the O 2 sensor (56) for the predetermined period when the air-fuel ratio control is performed.