A class of homogeneous isotropic space-time models including pseudo-Euclidean space as a special case is considered. Such a model is chosen, where the particle motion is described in the most adequate way. It means that the world tubes of all free particles (both classical and quantal) are characteristic geometrical structures of the space-time. The world function σ of this model has the form σ=σE+σ0, σE>σ0 where σE is the world function of the pseudo-Euclidean space, σ0 is a constant responsible for quantum effects. It is proportional to Planck’s constant ℏ.