  The spin states of Co$^{3+}$ ions in perovskite-type LaCoO$_3$, governed by
complex interplay between the electron-lattice interactions and the strong
electron correlations, still remain controversial due to the lack of
experimental techniques which can detect directly. In this letter, we revealed
the tensile-strain dependence of spin states, $i. e.$ the ratio of the high-
and low-spin states, in epitaxial thin films and a bulk crystal of LaCoO$_3$
via resonant inelastic soft x-ray scattering. The tensile-strain as small as
1.0% was found to realize different spin states from that in the bulk.
