A magnetic body detecting device constituting a magnet portion for magnetizing a magnetic body from a magnet main body portion, and a correcting portion which is disposed in front of magnet main body portion to correct a magnetic field generated by magnet main body portion, wherein the correcting portion is configured to form a specific position N having a desired magnetic field intensity by canceling out the magnetic field generated by magnet main body portion, and to adjust the magnetic field gradient at a magnetic field null point N of the magnetic field generated by magnet portion by causing magnet main body portion to be separated from a front end portion of magnet portion in accordance with the magnetic field gradient in the correcting portion, and wherein a magnetic sensor is disposed at the magnetic field null point N formed in the front end portion of the magnet portion.