Results of a narrow-band imaging for the redshifted [O II] lambda 3727 emission around a quasar at z = 4.7 obtained with the Subaru telescope and CISCO (a Cassegrain near infrared camera) are presented. A significant emission line is detected in the narrow-band H-2 [nu = 1-0 S(1)] filter at a location similar to 2." 4 northwest from the quasar, where the presence of a companion has been reported in Lyman cu emission and the rest-frame UV continuum. We identify this line as [OII] lambda 3727 emission and confirm that the source really is a companion at z = 4.7. The [OII] lambda 3727 flux from the companion is estimated to be 2.5 x 10(-17) erg s(-1) cm(-2). If the companion is a star-forming object, the inferred star-formation rate is as high as 45-230 M. yr(-1), even without assuming an extinction correction. This value is higher than those derived from the Lyman alpha emission or from the UV continuum. Thus, provided that the difference is caused by dust extinction, the extinction-corrected star-formation rate is calculated to be 45 to 2300 M. yr(-1), depending on the assumed extinction curves.