A device for detecting the sleepiness of a driver driving a vehicle is designed to determine if the resetting of threshold values for detection of sleepiness is performed based upon its necessity when an interruption of the driving occurs. The inventive device computes successively an eyelid opening/closing characteristic amount from time intervals between transitions between an opened state and a closed state of an eyelid detected successively; sets threshold values using the eyelid opening/closing characteristic amounts obtained in a predetermined period; and judges the driver feels sleepiness when the eyelid opening/closing characteristic amount deviates from a range defined with the threshold values. In restarting the driving after its interruption, it is judged if the resetting of the threshold values is to be performed based on the driver's condition till then, and the resetting is performed, using eyelid opening/closing characteristic amounts obtained after the restarting of the driving.