1. A lane-marker recognition system installed in a vehicle, comprising: an image pickup unit that picks up an image of a target region including a road ahead of the vehicle; a light-intensity detecting unit that detects a change in a light intensity of the target region in the picked-up image; a lane-marker recognizing unit that compares the change in the light intensity of the target region in the picked-up image with a predetermined threshold value, and recognizes a region of at least one lane marker in the target region based on a result of the comparison, the at least one lane marker being formed on the road; and a re-determining unit that re-determines the threshold value based on a light intensity of the recognized region of the at least one lane marker.