An object of the present invention is to provide a parking assistance system capable of intuitively and easily deciding operation points in the parallel parking. In the parking assistance system according to the present invention, a guide line (43), which is extended from a vehicle (20) in an oblique backward direction and indicates a boundary line used to decide whether or not the vehicle touches other vehicle (31) existing in front of a parking space (30) when parallel parking operations are started, and a guide line (51), which is separated backward from the guide line (43) and indicates another boundary line used to decide whether or not the vehicle touches other vehicle (53) existing at rear of the parking space (30) when the parallel parking operations are completed, are decided previously, and these guide lines as well as a backward picked-up image are displayed on a monitor screen in the parallel parking. Accordingly, the vehicle can be parked on the parking space (30) not to touch other vehicles (31), (53).