PURPOSE: To retain the sensitivity of an atomotive looped sensor constantly by employing the inductance of a loop coil laid on a road as the element of a resonator circuit of a tuned oscillator or providing to enable connection of a variable coil in parallel with the loop coil.

CONSTITUTION: When a vehicle exists on a detecting loop coil 1, its inductance L0 is reduced to increase the frequency of a tuned oscillator 5. Then, N-times multiplying circuit 6 is provided to raise the resolution with the frequency f1 set to Nf1. In this manner, when the vehicle rises on the loop coil 1, the inductance L0 is abruptly reduced to increase the oscilating frequency f0 of the tuned oscillator 5 to f1 and to simultaneously increase the output of the multiplying circuit 6 from Nf0 to Nf1. This output is measured by a pulse number counter 7 during the sampling gate time T outputted from the sampling pulse generator 8, and the result is calculated by a calculator 11.