An eye state detection apparatus includes a camera, a first calculator, a memory, a second calculator, and a third calculator. The camera obtains a plurality of face images of a driver. The first calculator calculates an opening amount of an eye of the driver based on each face image. The memory stores the opening amounts calculated by the first calculator. The second calculator groups the opening amounts into a plurality of groups in a sequential manner, calculates a group distribution of each group, calculates an entire distribution of all of the opening amounts, and sets the entire distribution as a reference distribution when a difference among the group distributions is within a predetermined range. The third calculator calculates an opening degree of the eye based on the reference distribution of the opening amounts when the reference distribution of the opening amounts is calculated by the second calculator.