PURPOSE: To detect the distance to an object from its own vehicle and relative speed of the object against its own vehicle without using any costas loop, etc.

CONSTITUTION: A delayed PN(pseudo noise) code delayed against a PN code used for phase modulation at the time of transmission is generated by means of a PN generator 28 and delay circuit 30. Received signals outputted from an reception antenna 30 are multiplied by the delayed PN code by means of a multiplier 26 after the frequency of the signals is converted into a first IF signal by means of a frequency conversion circuit 24. The output of the multiplier 26 is limited in band and, by discriminating the correlation of the signals obtained by limiting the band, the delaying amount corresponding to the time required for radio waves to go to and come back from an object from and to its own vehicle is detected. A distance detecting circuit 36 calculates the distance to the object based on the detected delaying amount. The output of the BPF 32 is further frequency-converted by means of another frequency conversion circuit 40 and converted into a voltage by means of an F-V converter 42. The output of the converter 42 contains a voltage component related to a Doppler frequency and a relative speed calculating circuit 44 calculates the relative speed of the object from the component related to the Doppler frequency.