1. A detecting apparatus for multi-lane vehicle detection, comprising: a radio-frequency transceiving unit comprising a signal input and a s signal output and generating a difference-frequency signal; at least two antennas of leakage-wave mode arranged closely to each other and coupled to said signal input and said signal output respectively; and an intermediate-frequency and digital signal processing unit coupled to said radio-frequency transceiving unit and generating at least a traffic parameter corresponding to at least a lane in response to said difference-frequency signal, wherein said antennas are configured to receive a microwave signal having a signal-to-noise ratio reflecting from a target where said signal-to-noise ratio is varied from 0 to 32 dB, and thereby said detecting apparatus is operable in said multi-lane vehicle detection.