1. An in-vehicle white line recognition apparatus for recognizing a white line, comprising: an on-board camera configured to capture an image of a surrounding of a vehicle; a white line candidate extraction section configured to perform image processing on the captured image to extract a plurality of white line candidates from the captured image, the plurality of white line candidates including a first white line candidate and a second white line candidate; at least one certainty factor calculation section configured to calculate a certainty factor of each white line candidate, the certainty factor indicating a degree of certainty that the each white line candidate represents the white line; and a white line selection section configured to select the white line to be recognized, based on the calculated certainty factors, wherein: when the first white line candidate and the second white line candidate satisfy a predetermined condition that (i) the first white line candidate and the second white line candidate are within a predetermined range and (ii) a ratio of contrast of the first white line candidate to contrast of the second white line candidate is less than a predetermined value, the certainty factor calculation section decreases the certainty factor of the first white line candidate as compared with when the first white line candidate and the second white line candidate do not satisfy the predetermined condition; the at least one certainty factor calculation section is a plurality of certainty factor calculation sections each calculating the certainty factor; the white line selection section calculates a likelihood of each white line candidate through multiplication of the certainty factors that are respectively calculated for the each white line candidate by the plurality of certainty factor calculation sections; the white line selection section selects one white line candidate having a largest likelihood from the plurality of white line candidates; and one of the plurality of certainty factor calculation sections is configured to calculate the certainty factor of each white line candidate, based on a contrast ratio of the each white line candidate with respect to a road, the road excluding a portion of the road corresponding to the each white line candidate.