1. An own-position estimation device comprising: a landmark position detector mounted in a vehicle and configured to detect positions of landmarks present around the vehicle relative to the vehicle; a movement amount detector configured to detect a movement amount of the vehicle; a landmark position accumulator configured to accumulate the positions of the landmarks relative to the vehicle detected by the landmark position detector as landmark position data by coordinates based on the movement amount detected by the movement amount detector; a straight line extractor configured to extract straight lines from the landmark position data accumulated at a plurality of times in the landmark position accumulator; a landmark position selector configured to select pieces of the landmark position data based on angles formed by intersecting ones of the straight lines extracted by the straight line extractor; a map information storage configured to store map information including positions of landmarks; and an own-position estimator configured to estimate a position of the vehicle by matching the pieces of the landmark position data selected by the landmark position selector and the positions of the landmarks in the map information.