We propose here a new coarsening mechanism ("diffusion-coupling mechanism") of droplet phase separation. For a system of high droplet density, the inhomogeneous distribution of droplets and the resulting anisotropic, selective coupling between the diffusion fields around neighboring droplets lead to the formation of the closed iso-concentration lines including more than two droplets. The excess energy associated with the concentration gradient in the matrix phase around neighboring droplets tries to reduce the length of the iso-concentration lines and causes the attractive interaction between the droplets. This mechanism could accelerate the droplet coarsening in spinodal decomposition.