In sliding friction of elastomers, the effective static friction coefficient μ_s^eff observed is not necessarily equal to the inherent real one μ_s possessed by the material; usually μ_s^eff < μ_s because of the elastic deformations of sliding objects and the propagation of shear fracture in the contact area. This indicates that focusing on elastic deformation of sliding objects can derive a new criterion of μ_s^eff for machine design. In this study, mainly through numerical simulations for a physical model assured by experiments in which measuring frictional force and observing contact area were carried out for a longitudinal line contact of polymethyl methacrylate, it is found that the stiffness ratio κ = K_b^tot / K_c^tot (where K_b^tot and K_c^tot are the total bulk stiffness and total tangential contact stiffness of the elastic sliding object, respectively) dominates the value of μ_s^eff.