A marking line detection system includes an imaging device, a marking line candidate detection unit, a marking line correction unit, a center line determination unit and a validity determination unit. The marking line candidate detection unit detects first and second vehicle's marking line candidates. The marking line correction unit corrects a position of the second vehicle's marking line candidate based on a position of the first vehicle's marking line candidate. When the first vehicle's marking line candidate is the valid center line, the marking line correction unit performs a correction of the position of the second vehicle's marking line candidate based on the position of the first vehicle's marking line candidate. When the first vehicle's marking line candidate is not the valid center line, the marking line correction unit does not perform the correction based on the position of the first vehicle's marking line candidate.