A parking assistance device includes: a route information storage unit storing in advance route information indicating a travel route at the time of parking, the route information being stored according to the forward start position; an inclination calculation unit configured to calculate a current inclination of a vehicle length direction of the vehicle with respect to the target parking position; an offset amount setting unit configured to set an offset amount in the interval direction of the forward route between the current vehicle position and the position at which the vehicle length direction is parallel with the forward route; and a guiding unit configured to guide the vehicle to the target parking position based on a travel route adjacent to the position at which the vehicle length direction is parallel with the forward route.