We have developed a microscopic blood model, that can express the deformation of an erythrocyte in micro-circulation, based on the Smoothed Particle Hydrodynamics (SPH) method. In the method, plasma fluid is discretized by SPH particles and an erythrocyte is modeled by internal SPH particles wrapped in elastic membrane particles. For verifying the model, in two dimensions, we numerically analyzed a popular phenomenon of erythrocyte; the tank tread motion of an erythrocyte under a constant shear field. The numerical results agreed well with the experimental data and the tank tread motion of erythrocyte was well realized by the model. From these results, it is confirmed that our blood model based on the SPH method can well express microscopic and rheological properties of erythrocytes.