Provided are ultra-low-field nuclear-magnetic-resonance myocardial electrical activity detection method and an ultra-low-field nuclear-magnetic-resonance device. The ultra-low-field nuclear-magnetic-resonance device includes magnetic shielding room; a high-sensitivity magnetic field sensor disposed adjacent to a measurement target disposed inside the magnetic shielding room; and a bias magnetic field generating coil for providing an external measurement bias magnetic field, corresponding to a proton magnetic resonance frequency (nuclear magnetic resonance frequency) corresponding to a frequency of periodic myocardial activity of a lesion desired to be measured, to the measurement target. The high-sensitivity magnetic field sensor measures a magnetic resonance signal generated from the measurement target.