Position detection of a medical device is prevented from being impossible even when the frequency characteristic of a magnetic induction coil is varied in accordance with the state of an external magnetic field for guiding the medical device. A medical device position detecting system is a medical device position detecting system which is inserted into the body of a subject and guided by an external magnetic field, and it is equipped with a resonance circuit that is mounted in the medical device, contains a magnetic induction coil having a magnetic material inside and generates an alternate magnetic field, an alternate magnetic field detecting device that is disposed at the outside of an operation region of the medical device and detects the alternate magnetic field generated by the magnetic induction coil, a position information calculator for calculating position information of the medical device on the basis of the detected alternate magnetic field, and a frequency setting unit for setting at least one of the frequency of the alternate magnetic field detected by the alternate magnetic field detecting device and the frequency of the alternate magnetic field generated by the magnetic induction coil on the basis of at least one of the intensity and direction of an external magnetic field at the position of the magnetic induction coil.