such as is shown in fig2 a is outputted into the amplifier 12 as can be appreciated from fig2 a the change in frequency period of the doppler signal is so small that the increment of change of the output voltage signal a from the period to voltage converter 15 for each period thereof is relatively small as shown in fig3 therefore the level of the differentiated signal b of the differentiator 17 does not exceed the second predetermined threshold value r 2 but does exceeds the first predetermined threshold value r 1 as shown in fig3 consequently the r s f f 19 outputs the high level voltage signal c as shown in fig3 to the and gate circuit 20 at this time if the vehicle halt detector 14 the comparator 16 and the amplitude judging circuit 13 all output high level voltage signals the and gate circuit 20 will output a high level voltage signal to the alarm circuit 30 when the r s f f 19 outputs the high level voltage signal c as a result the alarm circuit 30 is actuated to notify the driver that the other vehicle in front of the vehicle is moving away therefrom on the other hand when a pedestrian walks across the front detection area of the doppler radar a doppler signal such as is shown in fig2 b is outputted from the doppler radar module 11 as appreciated from fig2 b the rate of change of frequency is abrupt and the change in the period is so large that the output voltage signal a of the period to voltage converter 15 changes abruptly as the period changes as shown in fig4 therefore the level of the differentiated signal b of the differentiator 17 exceeds both.