The aim of this study is to clarify the relation between mental workload and the useful field of view in elderly. To examine the mental workload relative to the size of the useful field of view, an experiment was conducted with 24 participants (group 1: ages 22–24, group 2: ages 60–75). In the primary task, participants responded to visual markers appearing in a computer display. The useful field of view and the results of the secondary task for mental workload were measured. In the mental workload task, participants solved numerical operations designed to increase the mental workload. The experimental conditions in this task were divided into three categories (Repeat Aloud, Addition, and No Task), where No Task meant no mental task was given. The mental workload was changed in a stepwise manner. The quantitative assessment confirmed that the useful field of view narrows with the increase in the mental workload.