Pupil size is modulated not only by the luminance at the eye position but also by that at the attended location. This study aims to examine whether pupil changes also correspond to the luminance at the spatial location to which the attention is shifted in optokinetic nystagmus. The test stimulus consisted of randomly positioned dots that moved to the left or to the right on a display screen that was bright on one side of the centerline and dark on the other. The results show that pupil size changes in accordance with the luminance at the location to which participants' attention shifts as a result of optokinetic nystagmus (i.e., eye movements in the direction opposite to that of the motion stimulus). This study suggests that pupil size is modulated by the luminance at the location to which attention shifts through unidirectional field motion.