a device for detecting a line of sight includes an image pick - up element for picking up an eyeball image of a user , a line of sight detection circuit for detecting the line of sight of the user by calculating an output from the image pick - up element , a maximum value detection circuit for detecting a maximum output value output from the image pick - up element , a comparison circuit for comparing the maximum value with a predetermined threshold value , and an inhibition circuit for inhibiting a line of sight detection operation of the line of sight detection circuit when the maximum value is smaller than the threshold value .