Underground electromagnetic exploration method

In an underground electromagnetic exploration method using a transmission source (2) for generating induced current in order to periodically generate an induced current, magnetic observation comprises an observation step for observing a magnetic field based on smoke rings by using a magnetic field sensor device (1) that includes a sensor unit which includes a magnetic impedance element having a magnetic amorphous structure and that has a rod-shaped core part which guides the magnetic field to the magnetic amorphous structure in the length direction of the magnetic amorphous structure and a storage step for storing the magnetic field observation data that include magnetic field signals based on the output of the transmission source (2) for generating induced current. The magnetic observation is carried out repeatedly at multiple measurement points provided on the ground surface. The underground specific resistance distribution is calculated based on the observation data at each measurement point.