In this study investigated the effect of the frame design of a simple smartphone HMD on the stereoscopic vision and considered the design requirements for comfortable viewing environment. We mainly focused on the lens spacing used in screen enlargement and extension of the focal length. To investigate the differences in the fusional limit attributable to lens spacing, three HMDs with left/right eye-lens spacing of 57.5, 60, and 62.5 mm were utilized. When the three types of HMD and display were compared, the positive and negative direction fusional limits were closer than the display for all HMDs. In particular, that of 62.5 mm condition was shifted to significantly proximal in comparison with the control condition. The results showed a trend that the fusional range becomes nearer in a simple HMD. © 2016, Society for Imaging Science and Technology (IS&T).