An alertness level detection apparatus, which is equipped to a vehicle and detecting alertness level of a driver, includes a stimulus information acquirer detecting stimulus applied to driver's visual sensation within driver's visual field under a state in which the driver watches in travel direction of the vehicle and acquiring stimulus information related to occurrence time and occurrence position of the stimulus within the driver's visual field, a face image generator generating a face image of the driver, a line-of-sight position detector detecting line-of-sight position of the driver by analyzing the face image, a moving time duration detector detecting moving time duration taken for the driver to move the line of sight to the occurrence position of the stimulus immediately after the occurrence time of the stimulus, and an alertness level determiner determining the alertness level of the driver based on the moving time duration.