When the low magnetic field is applied to the synunetry axis of the axial synunetric electric field gradient tensor of the quadrupolar nucleus in the crystal with nuclear spin, I=1, two split resonance frequencies are observed by the Zeeman effect on the nuclear quadrupole resonance. The applied magnetic field is proportional to the difference of the two resonance frequencies and this proportionality constant for the $^{14}N$ of ${(CH_{2})}_{6}N_{4}$ single crystal is 0.16 mT/kHz. The NQR spectrometer is interfaced with a personal computer from which the resonance signals are displayed and the value of magnetic field is obtained directly from the difference of the two resonance frequeocies. The lowestest measured magnetic field was 0.20 mT using this NQR technique.