PURPOSE: To increase the accuracy of the detection level and to enable to change the detection level, by detecting the modulated light reflected at the mobile object and by performing A/D conversion for it.

CONSTITUTION: The light modulated with the output of the oscillation circuit 1 is projected on the object 3 from the projection section 2 and it is reflected. This is received at the photo detection section 4, and it is amplified 5 and detected 6. This output is inputted to the A/D converter 8 constituted with a plurality of bits via the level adjusting section 7 to convert the signal into binary signal. The circuit 9 is so constituted that output 12 is made when any part of the circuit 9 has output via the OR circuit 10 by inputting the output of all the bits of the said converter 8 from the upper rank bit respectively. Thus, by using the converter 8 having more number of bits, the accuracy of detection can be increased and also, the detection level can be changed with the number of bits connected to the circuit 9.