[en] In the laboratory of the authors, the hyperfine structure interaction in various matters such as ionic crystals, semiconductors and metals has been studied by using short life, light, β radioactive nuclei including ¹²B and ¹²N as the probe. In this paper, the experimental results on the second implantation position of ¹²B and ¹²N in Mg single crystals are reported. When ¹²B or ¹²N was implanted in Mg, two pairs of the NMR lines have been observed. For the detailed studies on the angle dependence of the second sites and others, the former method was not suitable, therefore, by using the new nuclear quadrupole resonance (NNQR) method, the properties of the electric field gradient distribution at the second sites were studied. The experimental method by applying the NNQR method is explained. In the observation of two samples, the resonance line for the second site was not observed in one of them. As for the properties of the sample, in which the second site was observed, the electric field gradient at the second site was perpendicular to the sample surface and c-axis, and the distribution spread on the whole crystal, and was not affected by the surface structure. It has been known that ¹²N is implanted at the triangular column position in Mg. (K.I.)