A determining method for detecting obstacles on and around a parking path that a vehicle is to move on, the method comprising: determining whether an ultrasonic noise exists in time of flights TOFs of ultrasonic waves reflected by an object (X,Y) and received, wherein, when a sensor emitting the ultrasonic wave and a sensor receiving the ultrasonic wave are the same, the TOF of the ultrasonic wave received by the sensor is referred to as direct wave TOF, and when a sensor emitting the ultrasonic wave and a sensor receiving the ultrasonic wave are different, the TOF of the ultrasonic wave received by the sensor is referred to as indirect wave TOF; generating a virtual object on an outline of the parking path based on a received ultrasonic wave time of flight TOF when no ultrasonic noise exists, wherein the virtual object is generated at an intersection (P_virtual) in which a circle having a center at a position of a sensor emitting and receiving a direct wave and a radius of a direct wave hTOF crosses the outline of the parking path; wherein direct wave hTOF = direct wave TOF / 2; generating a virtual indirect wave TOF using the virtual object, wherein the virtual indirect wave TOF is calculated as a sum of a straight line connecting a position of the sensor emitting and receiving the direct wave to the virtual object and a straight line connecting the virtual object to a position of a sensor receiving the indirect wave; and determining whether the object is located inside or outside the outline of the parking path by comparing a real indirect wave TOF, which is an indirect wave TOF among the received ultrasonic wave TOFs, to the virtual indirect wave TOF. determining whether an ultrasonic noise exists in time of flights TOFs of ultrasonic waves reflected by an object (X,Y) and received, wherein, when a sensor emitting the ultrasonic wave and a sensor receiving the ultrasonic wave are the same, the TOF of the ultrasonic wave received by the sensor is referred to as direct wave TOF, and when a sensor emitting the ultrasonic wave and a sensor receiving the ultrasonic wave are different, the TOF of the ultrasonic wave received by the sensor is referred to as indirect wave TOF; generating a virtual object on an outline of the parking path based on a received ultrasonic wave time of flight TOF when no ultrasonic noise exists, wherein the virtual object is generated at an intersection (P_virtual) in which a circle having a center at a position of a sensor emitting and receiving a direct wave and a radius of a direct wave hTOF crosses the outline of the parking path; wherein direct wave hTOF = direct wave TOF / 2; generating a virtual indirect wave TOF using the virtual object, wherein the virtual indirect wave TOF is calculated as a sum of a straight line connecting a position of the sensor emitting and receiving the direct wave to the virtual object and a straight line connecting the virtual object to a position of a sensor receiving the indirect wave; and determining whether the object is located inside or outside the outline of the parking path by comparing a real indirect wave TOF, which is an indirect wave TOF among the received ultrasonic wave TOFs, to the virtual indirect wave TOF.