An object of the present invention is to provide an eye opening degree detection system that can accurately and stably calculate an eye opening degree. The eye opening degree detection system includes imaging devices that generate images including regions of both eyes, a one eye opening degree calculation unit that calculates each one eye opening degree of the left and right eyes of a first image, an eye opening degree selection unit that selects a correctly-calculated one eye opening degree, an eye opening degree calculation unit that calculates an eye opening degree on the basis of the one eye opening degree and an eye opening degree determination unit that compares the eye opening degree calculated on the basis of the first image with an eye opening degree calculated on the basis of a second image prior to the first image to determine the propriety of the eye opening degree.