Diplopia, a common symptom in spinocerebellar ataxia 3/Machado-Joseph disease (SCA3/MJD) cases, is not always due to asymmetric ophthalmoplegia. We found a Japanese SCA3/MJD family, in which three patients clearly had an impairment of divergence eye movement. We thus quantitatively examined the vergence ranges in eight Japanese SCA3/MJD cases using the synoptophore test. An impairment of the vergence eye movements was found in all patients, and the vergence impairment pattern, but not the ophthalmoplegia pattern, was found to be compatible with the diplopia pattern. The diplopia in SCA3/MJD cases is, therefore, attributed, at least in part, to the impairment of the vergence eye movements.