The individual decay channels of samarium L-subshell vacancy states were experimentally investigated by the synchrotron photoionization method. For the Coster-Kronig yield ${\mathit{f}}_{13}$, i.e., the relative contribution of the nonradiative ${\mathit{L}}_{1\mathrm{\ensuremath{-}}}$${\mathit{L}}_{3}$ channel to the total decay of ${\mathit{L}}_{1}$-vacancy states, the value ${\mathit{f}}_{13}$=0.19\ifmmode\pm\else\textpm\fi{}0.03 was obtained. This is a surprisingly small value regarding both prediction from theory (e.g., ${\mathit{f}}_{13}$=0.32 according to Chen, Craseman, and Mark [Phys. Rev. A 24, 177 (1981)]) and extrapolation from neighboring elements (${\mathit{f}}_{13}$\ensuremath{\approxeq}0.32 for the elements with Z=72--74 according to our previous work [U. Werner and W. Jitschin, Phys. Rev. A 38, 4009 (1988)]).