There have been several recent attempts to aid car drivers by providing information on internal and external car environments. The optimal amount of information must be determined to avoid confusion. In this study, the “Stroop task” was used for information processing, and the cognitive load was gradually increased by adding information in stages. We designed and conducted two tasks that originate in the “Stroop task”; these two tasks feature significant differences in cognitive load. We also measured brain activity using near-infrared spectroscopy (NIRS) under the assumption that such activity can be used as an index of cognitive load. Both tasks were associated with increased oxy-hemoglobin levels in the prefrontal area, and the task with a higher cognitive load was associated with a more substantial increase in oxy-hemoglobin; this indicates that oxy-hemoglobin levels may be used as an objective index for the evaluation of information-associated cognitive load.