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In the recording of magnetocardiogram (MCG), the largest environmental magnetic field interference is the line-frequency noise. Even with multi p-metal layer shielding, 50 or 60 Hz interference is not completely rejected. In this paper, we report an extension of active noise cancellation to the magnetic field interference, in which a digital signal processor (DSP) was used. A sinusoidal wave generated in the DSP was filtered adaptively in the least mean square algorithm, and its output was supplied as cancellation field by means of a coil that was wound around a small room magnetically shielded with a single g-metal wall. With the help of the low-frequency noise compensation, which was also applied to the shielded room, wide band MCG recording was possible inside the room using a high temperature superconducting quantum interference device.