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A biomagnetic measurement system on mice using a dc superconducting quantum interference device magnetometer has been developed. We are going to use it for comparative magnetoencephalogram (MEG) and magnetocardiogram (MCG) studies of transgenic mice to clarify the functional influence of gene modification in a living body. The system has a low-Tc direct coupled dc SQUID. A focuser type pickup coil whose outside/inside diameter is 1 mm/100 μm was adopted for detailed functional mapping of the brain or the heart of mice. The minimum lift-off distance was reduced to 700 μm to achieve the sufficient spatial resolution and magnetic field sensitivity. The field sensitivity of this system is 1.3 pT/Hz12/ in the white-noise region (10 Hz-10 kHz). The MCG of a wild mouse at 9-months of age was measured and compared with the data of 3-month old mice. The spatial and time changes of MCG signals were similar in both ages of mice, while the amplitude of MCG signals of 9-month mice was less than 40% of that of 3-month mice. This decrease in MCG amplitude may be due to aging. The result suggested the capability of our system in evaluating the mice MCG signals.