An other lane monitoring device of the present disclosure includes an other vehicle acquisition section, a path acquisition section, a lane estimation section, and a position estimation section. The other vehicle acquisition section is configured to acquire other vehicle position information that indicates the current position of an other vehicle near the own vehicle. The path acquisition section is configured to acquire an own vehicle travel path that indicates a path along which the own vehicle has traveled. The lane estimation section is configured to use the own vehicle travel path as a basis for estimating an other lane area that is a lane where the other lane is present, the other lane being different from an own lane where the own vehicle is present. The position estimation section is configured to estimate a future position of the other vehicle indicated by the other vehicle position information, assuming that the other vehicle moves along the own vehicle travel path or in the other lane area.