1. A drowsiness determining device comprising: a sympathetic parameter acquiring unit for acquiring a parameter concerning a sympathetic nerve of a subject; a parasympathetic parameter acquiring unit for acquiring a parameter concerning a parasympathetic nerve of the subject; a sympathetic increase determining unit for determining whether the sympathetic parameter acquired by the sympathetic parameter acquiring unit is greater than a sympathetic threshold or not; and a drowsiness determining unit for determining drowsiness of the subject according to an increase/decrease relationship between the sympathetic parameter acquired by the sympathetic parameter acquiring unit and the parasympathetic parameter acquired by the parasympathetic parameter acquiring unit when the sympathetic increase determining unit determines that the sympathetic parameter is greater than the sympathetic threshold.