An inattentive state determination device includes an observed amount acquiring means for acquiring information on the driver's line of sight and at least one of either information on driving operations or information on the traveling state as an observed amount; and an inattentive state determining means for determining the inattentive state of the driver based on the stochastic relationship between the inattentive state and the observed amount as well as the observed amount acquired by using the observed amount acquiring means. In addition, the inattentive state determining means calculates the probability that the driver is driving inattentively and determines that the driver is driving inattentively when the calculated probability exceeds a threshold.