A data processing device that performs data processing of monitoring a person, the data processing device includes: a calculator configured to calculate pupil movement and head movement of the person； an evaluator configured to evaluate a suitability degree of a situation in calculating the VOR； a provision unit configured to provide the suitability degree evaluated by the evaluator to data calculated by the calculator； a selector configured to select a first technique of calculating the sleepiness based on the VOR of the person or a second technique different from the first technique based on the suitability degree provided to the data； a first sleepiness calculator configured to calculate the sleepiness based on the first technique when the selector selects the first technique； and a second sleepiness calculator configured to calculate the sleepiness based on the second technique when the selector selects the second technique.
