1. A parking assistance device comprising: a surrounding image presenting section configured to receive a captured image of at least a view in a direction of movement of a vehicle taken by a car-mounted camera and allow a surrounding image of the vehicle to be displayed on a monitor device inside the vehicle; a parking target setting section configured to set a parking target of the vehicle; a route computing section configured to compute a guide route for allowing the vehicle to move forward to a reverse start position and then reverse to the parking target and the reverse start position based on the parking target with a predetermined tolerance included; and a graphic control section configured to generate forward guide markers constituted by a marker group including a far marker corresponding to the reverse start position and a near marker that represents an area via which the vehicle can reach the reverse start position and is located closer to the vehicle than the far marker, the forward guide markers indicating a forward route to the reverse start position by an arrangement of the marker group the width of which is increased toward the near marker from the far marker, and superimpose the markers on the surrounding image.