PROBLEM TO BE SOLVED: To provide a method for detecting a low frequency signal, which eliminates electromagnetic wave noise at low frequencies picked up from an ambient environment and a power line to detect a low frequency signal when detecting the low frequency signal by a sensor coil.SOLUTION: Two sensor coils 5 and 6 are arranged so that their rear surfaces are matched with each other, and a difference between their detection signals is obtained and is subjected to signal processing, whereby electromagnetic wave noise at low frequencies is eliminated. A magnetostatic field is generated by a magnet booster 11, and the sensor coils 5 and 6 are arranged within the magnetostatic field, and variation or disturbance of the magnetostatic field is detected by the sensor coils 5 and 6. A signal processing unit amplifies and rectifies signals detected by the sensor coils 5 and 6. The rectified signal of the sensor coil 6 has the phase inverted and is added to the rectified signal of the sensor coil 5, whereby noise
