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 vestibulo-ocular reflex movement based on the pupil movement and the head movement of the person; a provision unit configured to provide the suitability degree evaluated by the evaluator to data relating to the pupil movement and the head movement of the person calculated by the calculator; and a reflex movement calculator configured to calculate the vestibulo-ocular reflex movement of the person based on the data in consideration of the suitability degree.