The purpose of this study was to report a patient with Oguchi disease whose ophthalmological characteristics were masked by retinitis pigmentosa (RP). The method used in this study was case report. A 53-year-old man had a progressive decrease in his visual acuity and was diagnosed with RP because of night blindness, fundoscopic findings, ring scotoma, and extinguished single-flash electroretinograms (ERGs). However, a faint golden-yellowish reflex of the retina prompted us to make a more detailed examination of the fundus after a long period of dark adaptation, ERGs, and genetic analysis. Examinations showed the Mizuo-Nakamura phenomenon, relative intact photopic ERGs, and a SAG mutation, and the patient was diagnosed with RP associated with Oguchi disease. When RP accompanies Oguchi disease, the clinical characteristics of Oguchi disease might be masked. In such a situation, the correct diagnosis is difficult. However, careful analysis of clinical findings will suggest Oguchi disease, which can be confirmed by molecular genetics.