A parking assist method comprising: acquiring recognition information provided by ranging sensors about a plurality of parked vehicles; setting a virtual parking frame group in which a parking frame group is virtualized, the parking frame group comprising a plurality of parking frames; selecting representative points set to centers, centers in a vehicle width direction, or centers in a vehicle longitudinal direction of the recognized plurality of parked vehicles; wherein the virtual parking frame group comprises a plurality of adjacent virtual parking frames that have a same size and are arranged side by side or parallel along a predetermined straight line; and the method further comprising optimizing a position of the virtual parking frame group by aligning center lines of the virtual parking frames of the virtual parking frame group with respect to the representative points of the parked vehicles, so that all of the distances between positions of the representative points and the center lines of the virtual parking frames closest to those positions are less than a predetermined value for all the representative points and all the virtual parking frames, by minimizing the distances, the virtual parking frames overlapping the parked vehicles; determining the aligned virtual parking frame group as the parking frame group comprising the plurality of parking frames; and selecting, from among the plurality of parking frames included in the determined parking frame group, the parking frames in which the recognized parked vehicles do not exist as selection candidates of the parking frames for a target parking position. acquiring recognition information provided by ranging sensors about a plurality of parked vehicles; setting a virtual parking frame group in which a parking frame group is virtualized, the parking frame group comprising a plurality of parking frames; selecting representative points set to centers, centers in a vehicle width direction, or centers in a vehicle longitudinal direction of the recognized plurality of parked vehicles; wherein the virtual parking frame group comprises a plurality of adjacent virtual parking frames that have a same size and are arranged side by side or parallel along a predetermined straight line; and the method further comprising optimizing a position of the virtual parking frame group by aligning center lines of the virtual parking frames of the virtual parking frame group with respect to the representative points of the parked vehicles, so that all of the distances between positions of the representative points and the center lines of the virtual parking frames closest to those positions are less than a predetermined value for all the representative points and all the virtual parking frames, by minimizing the distances, the virtual parking frames overlapping the parked vehicles; determining the aligned virtual parking frame group as the parking frame group comprising the plurality of parking frames; and selecting, from among the plurality of parking frames included in the determined parking frame group, the parking frames in which the recognized parked vehicles do not exist as selection candidates of the parking frames for a target parking position.