A biomagnetic measurement system was developed, suitable for the detection of magnetospinogram (MSG) and magnetocardiogram (MCG) signals from the dorsal surface of supine subjects. It is effective for noninvasively observing the electric activity of the spinal cord and/or heart. These biomagnetic signals are extremely weak, and magnetic flux sensors based on superconducting quantum interference devices (SQUIDs) are necessary to detect them. However, highly sensitive magnetic field measurement often suffers from ultra low-band circumstance noise mainly caused by transportation in urban areas. We applied reference sensors for monitoring the circumstance noise, and their outputs multiplied by appropriate weight coefficients were directly input to the feedback coil of a SQUID gradiometer. Synthesized in-phase components reduced the ultra low-band noise by approximately 90%. Both the MSG and MCG signals were successfully detected in a moderately magnetically shielded room. Even though the MCG signal band overlapped the ultra low-band noise, the signal-to-noise ratio was improved.