This study investigated the effects of various task loadings on driving performance for drivers who used a touch screen. The objectives were to evaluate the influence of three secondary tasks on the primary task, to clarify the effects of the task loadings in combination on driving performance, and to determine effective measures for minimizing the effect of touch screen tasks on driving performance. To achieve these objectives, a field experiment was conducted on a test track with 16 participants. There were three secondary tasks: viewing numbers on the touch screen and reading them aloud in ascending order, viewing numbers on the touch screen and tapping them in ascending order on the touch screen, and tapping the four corners of the touch screen. Vehicle motions, the duration of the driver's glance at the screen, and the duration of the driver's tapping the screen were measured. The driving performance indicators were speed, headway distance, and lateral distance from the center of the lane. When drivers operated the touch screen while driving, each task loading was found to have a different effect on each driving performance indicator. The analysis found that measures that were effective in maintaining high driving performance decreased the individual duration of each secondary behavior and reduced the variety of task loadings and their combinations. Driving style also influenced the degree to which each task loading affected driving performance.