An inattention determination device includes a sight direction detecting unit that detects a sight direction of a driver, and an inattention determining unit that measures an inattention continuation time from a time point when the sight direction is a predetermined inattention direction to a time point when the sight direction deviates from the predetermined inattention direction and becomes a direction other than the inattention direction and determines that the driver is inattentive when the intention continuation time is equal to or more than a predetermined time. The inattention determining unit maintains measurement of the inattention continuation time when the sight direction is changed at a speed higher than a predetermined movement speed from the predetermined inattention direction to a direction other than the inattention direction or when the sight direction is momentarily a direction other than the inattention direction.