This study proposes a submenu window as a navigation aid for mobile Internet access on a cellular phone. The submenu window presents child-level menu items along with their upper-level menu. The effectiveness of the submenu window was investigated in an experiment. The factors manipulated in the experiment include the types of submenu window, age of participants, and timing of presenting the submenu. Task completion time, number of button clicks, and subjective preference were measured while the participants were performing target search tasks. The results showed that they preferred the separate submenu window. The best performance was obtained when the submenu immediately followed the selection of its parent menu. In addition, the participants in their 20s found a target menu item with fewer button clicks than did the teenagers. The separate submenu window with no delay was recommended in terms of both performance and subjective preference. The design guidelines for submenu windows on a small-screen device are also provided. The results of this study are expected to be applicable to menu-driven interfaces of many small-screen devices such as digital cameras and personal digital assistants.