To provide a surveying method and a kind of system of distance by radar for car backing which can perform kinetic parameter adjustment, based on the surrounding environment, and a surveying system with a radar for car backing capable of exactly deciding whether obstacles exist and deciding the distance between an obstacle and a vehicle body, using the method.

A set of threshold used for distance decision is stored in one memory device, a multitude of samplings are performed to reflected signals obtained as the results of measurements with a plurality of sensors, then, each sampled value is compared with the threshold, and by using this, whether an obstacle exists behind the car and the distance between the obstacle and the car is decided. A central processor 10 has a plurality of sensors switched over through a passage selection unit 30, by which surveying signals emitted by each sensor or received reflection signals are controlled. When the sensor receives the signal, the reflected signal is converted into a digital signal with an A/D conversion circuit 50, the distance is decided with the central processor 10, by which the distance between the obstacle and the car is determined.