The object is to provide a drowsiness detecting device for detecting strong drowsiness to become a dozing state. A drowsiness detecting device comprises 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 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 and parasympathetic parameters when the sympathetic increase determining unit determines that the sympathetic parameter is greater than the sympathetic threshold.