1. A rangefinder apparatus comprising: AF data generating means for forming an image of light from an object to be subjected to rangefinding onto a pair of line sensors each including a plurality of light-receiving elements, and generating AF data for computing a correlation value according to signals obtained from the light-receiving elements; AF data acquiring means for acquiring the AF data from a pair of employed sensor areas used for rangefinding in the pair of line sensors; correlation value computing means for determining a pair of window areas for selecting the AF data to be used for computing the correlation value within the pair of employed sensor areas, and successively computing correlation values while shifting the pair of window areas; interpolated correlation extreme value computing means for detecting a correlation extreme value(s) among the correlation values computed by the correlation value computing means, and interpolating thus detected correlation extreme value(s) so as to compute an interpolated correlation extreme value(s); highest correlation value detecting means for detecting as a highest correlation value the interpolated correlation extreme value exhibiting the highest correlation among the interpolated correlation extreme value(s); shift amount computing means for computing a shift amount of the window areas yielding the highest correlation value; and object distance calculating means for calculating a distance to the object according to the shift amount computed by the shift amount computing means.