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There have been virtually no reports on the origin of environmental magnetic fields which act as noise sources in biomagnetic measurements, despite its importance to the design of magnetic shielded rooms and to the quality of signals obtained in biomagnetics experiments. In this paper, the measured variation of environmental magnetic fields at three locations-- Tokyo Denki University, Hokkaido University, and the Research Laboratory of Takenaka Corp.--is described, and the origin of magnetic field noise is discussed with reference to magnetic fields generated by electric trains. Near railways, large variations in magnetic field are observed due to imbalance between the feeder line and return rail line. Simulations confirmed that trains are especially prone to generate fields in the X-axis direction (perpendicular to the direction of train travel).