In order to offer a new and useful device in being able to objectively assess the psychological state when a subject performs an assignment, a psychological state assessment device is configured to irradiate a specified measurement site in the brain of a subject with near infrared light of a specified wavelength, to obtain time change data indicating time change data for the amount of blood and/or amount of blood components respectively while performing a task and while at rest, and to assess the psychological state of the subject while performing the assignment based on time change data during the rest period in comparison with time change data while performing the task.