1. A drowsiness determination apparatus comprising: a decreased wakefulness detection means for detecting a sign of decreased wakefulness due to drowsiness based on an eye opening degree; a struggle detection means for detecting a sign of struggle against drowsiness based on a distance between an eyebrow and the eye just below the eyebrow; and a drowsiness determination means for determining a drowsiness level based on the sign of decreased wakefulness and the sign of struggle, wherein the drowsiness determination means determines no drowsiness when the sign of decreased wakefulness is not detected, determines possession of drowsiness when the sign of decreased wakefulness is detected, and determines a drowsiness level according to presence or absence of the sign of struggle in a state where the possession of drowsiness is determined; the drowsiness determination means determines: a condition A where the eye opening degree is less than or equal to a first threshold value; a condition B where the eye opening degree is less than or equal to a second threshold value smaller than the first threshold value; and a condition C where the distance between an eyebrow and the eye just below the eyebrow is greater than a specified threshold value; and wherein the drowsiness determination means determines a drowsiness level as strong drowsiness when the condition B is satisfied; and wherein the drowsiness determination means determines the drowsiness level as medium drowsiness lower than the strong drowsiness when the condition A and the condition C are satisfied and the condition B is not satisfied; and wherein the drowsiness determination means determines the drowsiness level as weak drowsiness lower than the medium drowsiness when the condition A is satisfied and the condition B and the condition C are not satisfied; and wherein the drowsiness determination means determines the drowsiness level as no drowsiness when the condition A and the condition B are not satisfied.