This study examined the existence of functional hemispheric asymmetry in visual sustained attention in a dual-task setting. In the experiments, a circular arrangement of Gabor patches (GPs) was successively presented in the periphery during a 40-min vigil. The primary task was successive discrimination of digit type (Experiment 1) or tone frequency (Experiment 2). The secondary task was oddball orientation detection, requiring subjects to determine whether all of GPs were same or whether there was an odd one in the upper left, upper right, lower left, or lower right visual fields. Overall, results indicated that the perceptual sensitivity (A') for the detection task was higher in the left visual field than in the right visual field. Visual field differences in the vigilance decrement (the decline in A' over time) were observed when the primary task involved auditory discrimination. The implications for the functional lateral asymmetry in human sustained attention are discussed.