In this paper, we consider interferences from other automobile pulse radars using same frequency spectrum. In order to eliminate the interference, we propose the PN code modulation method. This method uses the cross-correlation between PN codes with different seed. The ROC performance is used for comparing the proposed detector to conventional method. And the proposed detector can decide the present or absent of targets and measure the range of the targets by using memory buffer of range gate. Especially, we use false alarm probability for all range gates. That is the false alarm if in any one range gate the false alarm occurs. From the simulation result, we can see that the proposed detector with using PN code is not influenced by interferences.