1. An elastic reverse-time migration system, comprising: a virtual source calculation unit being input seismic data and a subsurface structure model to calculate a virtual source; a virtual source wavefield calculation unit calculating a virtual source wavefield from the calculated virtual source; a back-propagated wavefield calculation unit loading the seismic data to perform back propagation processing and calculate a back-propagated wavefield; a virtual source wavefield waveform separation unit using stress-displacement relationships to decompose P-wavefield from the virtual source wavefield; a back-propagated wavefield waveform separation unit using the stress-displacement relationships to decompose P-wavefield from the back-propagated wavefield; and a convolution unit convoluting the two wavefields decomposed by the virtual source wavefield waveform separation unit and the back-propagated wavefield waveform separation unit, respectively, wherein the stress-displacement relationships satisfy the following Equation τ xx = ( λ + 2 ⁢ μ ) ⁢ ∂ u x ∂ x + λ ⁢ ∂ u z ∂ z ⁢ τ zz = ( λ + 2 ⁢ μ ) ⁢ ∂ u z ∂ z + λ ⁢ ∂ u x ∂ x (in the above Equation, λ and μ represent the Lame's parameters, τ xx and τ zz represent stress tensors, u x and u z represent displacements, respectively, for the x- and z-axes).