Provided is a navigation assistance device capable of obtaining an obstacle zone by target in consideration of an allowance. A navigation assistance device for obtaining an obstacle zone by target from a relationship between an own ship and a target ship includes a margin acquisition unit that acquires a margin indicating the degree of allowance, and an arithmetic unit that obtains a margin position which is a position separated from at least any of the bow or stem of the target ship in a target occupied space set with reference to the position of the target ship by a distance determined according to the margin, obtains a collision course based on a position determined according to the motion vector of the target ship with reference to the margin position, and calculates the obstacle zone by target based on the obtained collision course.