An inattention determining device comprising: an imaging unit adapted to capture an image of a face of a driver in a vehicle and to output a face image; a sight line direction detector adapted to detect a sight line direction of the driver from the face image and to output a sight line detection result; an inattention determining unit adapted to determine that the driver is in an inattention state when the sight line direction of the sight line detection result deviates from a predetermined range being a non-inattention determining range; an information acquiring unit adapted to acquire information of driving behavior of the vehicle including a turning direction of the vehicle and to output a driving behavior acquisition result; a range changing unit adapted to rotate the predetermined range from a first predetermined range to a second predetermined range by a predetermined angle (θ) on the basis of the driving behavior acquisition result; and a curve detector adapted to detect an existence of a curve to the front within a predetermined distance from the vehicle and a curve direction of the curve, and to output a curve detection result, wherein the inattention determining unit is adapted to determine whether or not the driver is in the inattention state on the basis of the second predetermined range, characterized in that the range changing unit is adapted to rotate, when the curve detection result is output from the curve detector, the predetermined range from the first predetermined range to the second predetermined range in the curve direction by the predetermined angle(θ), when the sight line direction of the driver deviates from the first predetermined range on the curve direction side, at the timing before the driving behavior of the vehicle is changed and the turning direction of the vehicle is changed to the curve direction. an imaging unit adapted to capture an image of a face of a driver in a vehicle and to output a face image; a sight line direction detector adapted to detect a sight line direction of the driver from the face image and to output a sight line detection result; an inattention determining unit adapted to determine that the driver is in an inattention state when the sight line direction of the sight line detection result deviates from a predetermined range being a non-inattention determining range; an information acquiring unit adapted to acquire information of driving behavior of the vehicle including a turning direction of the vehicle and to output a driving behavior acquisition result; a range changing unit adapted to rotate the predetermined range from a first predetermined range to a second predetermined range by a predetermined angle (θ) on the basis of the driving behavior acquisition result; and a curve detector adapted to detect an existence of a curve to the front within a predetermined distance from the vehicle and a curve direction of the curve, and to output a curve detection result, wherein the inattention determining unit is adapted to determine whether or not the driver is in the inattention state on the basis of the second predetermined range, characterized in that the range changing unit is adapted to rotate, when the curve detection result is output from the curve detector, the predetermined range from the first predetermined range to the second predetermined range in the curve direction by the predetermined angle(θ), when the sight line direction of the driver deviates from the first predetermined range on the curve direction side, at the timing before the driving behavior of the vehicle is changed and the turning direction of the vehicle is changed to the curve direction.