Electronic state of the organic salt ${\mathrm{}(\mathrm{D}\mathrm{I}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2$Ag, where DI-DCNQI is 2,5-diiodo-$N$, $N^{\prime }$-dicyanoquinonediimine

${\mathrm{}(\mathrm{D}\mathrm{I}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2$Ag where DI-DCNQI is 2,5-diiodo-$N$, $N^{\prime }$-dicyanoquinonediimine has been investigated by electrical resistance, magnetic susceptibility, and ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR measurements, and found to be nonmetallic with a charge gap of ∼490 K and with spin susceptibility showing localization of the $\pi$-electron spins. Antiferromagnetic ordering is evidenced at a low temperature. This is a first case of the electron antiferromagnet among the DCNQI-metal family in which the $\pi$ electrons on the DCNQI columns have an antiferromagnetic state. The nature of a Mott insulator demonstrates the importance of electronic correlation in the $\pi$ electrons in the DCNQI systems.