Patent ID: 7525308

Claim:
A magnetic detecting device comprising: an apply coil for generating an alternate magnetic field of variable frequency and a power source for the apply coil, a magnetic sensor for detecting a change in a magnetic field induced by a test object placed at a distance from a surface of said apply coil and toward said test object, said magnetic sensor being provided with a cancel coil for canceling a magnetic field generated at the position of the magnetic sensor by a direct magnetic field and/or said apply coil, a measurement circuit for the magnetic sensor for measuring a detection signal of said magnetic sensor, a lock-in amplifier circuit for detecting an output of said measurement circuit for the magnetic sensor into two signals having the same frequency as that of said apply coil and phases different from each other by 90 degrees and an analysis means for analyzing a phase change of the output of said magnetic sensor using an output signal from said lock-in amplifier circuit.