The radiations from 5.8-day ${\mathrm{Sb}}^{120m}$ have been investigated in detail, using sector-type double-focusing magnetic spectrometers and scintillation counters. The following measurements were performed: coincident gamma-ray spectra, directional correlations of 1.18 Mev $\ensuremath{\gamma}$---1.03 Mev $\ensuremath{\gamma}$, 1.03 Mev $\ensuremath{\gamma}$---0.20 Mev $\ensuremath{\gamma}$, 1.18 Mev $\ensuremath{\gamma}$---0.20 Mev $\ensuremath{\gamma}$, 0.20 Mev $\ensuremath{\gamma}$---0.09 Mev $\ensuremath{\gamma}$, 0.20 Mev ${\mathrm{Ke}}^{\ensuremath{-}}$ -0.09 Mev $\ensuremath{\gamma}$ and 0.20 Mev $\ensuremath{\gamma}$---0.09 Mev ${\mathrm{Ke}}^{\ensuremath{-}}$. The 1.18 Mev $\ensuremath{\gamma}$---1.03 Mev $\ensuremath{\gamma}$ polarization direction correlation, the conversion electron spectra, and the mean lives of the 2.41- and 2.50-Mev levels have also been investigated. The decay scheme of 5.8-day ${\mathrm{Sb}}^{120m}$ proposed by McGinnis, which gives the following assignments to the excited states in ${\mathrm{Sn}}^{120}$, 1.18 Mev(2+), 2.21 Mev(4+), 2.41 Mev(6+), and 2.50 Mev (7-), has been confirmed uniquely. These levels have been interpreted as arising from the neutron configurations ${(2{d}_{\frac{3}{2}})}^{2}{(1{h}_{\frac{11}{2}})}^{2}$ and ${(2{d}_{\frac{3}{2}})}^{3}{(1{h}_{\frac{11}{2}})}^{1}$. The lifetimes of the 2.41-, 1.18-, and probably 2.21-Mev levels are shorter by a factor 3-4 than those expected for single-proton transitions and may be explained by the neutron transition, assuming a somewhat larger effective charge of neutron in spherical nuclei. The high forbiddenness of the 0.09-Mev $E1+M2$ transition is ascribable to the simultaneous $j$- and $l$-forbidden transition between the states involving ${(2{d}_{\frac{3}{2}})}^{3}{(1{h}_{\frac{11}{2}})}^{1}$ and ${(2{d}_{\frac{3}{2}})}^{2}{(1{h}_{\frac{11}{2}})}^{2}$ configurations.No appreciable effects of the nuclear finite size or any nuclear structure dependence on the internal conversion process have been detected even for the highly forbidden 0.09-Mev $E1+M2$ transition.In the decay of 16-min ${\mathrm{Sb}}^{120}$, the 1.18-Mev gamma ray has been detected. However, other gamma rays expected from the second level multiplet, which is a characteristic feature of the vibrational model, have not been found.