We present a new direct simulation technique of inertialess particle suspensions in planar elongational flow of a Newtonian fluid. The extensional bi-periodic domain concept is introduced such that a single cell problem with a small number of particles may represent a large number of repeated structures of such a cell in planar elongational flow. For implicit treatment of the hydrodynamic interaction between particles and fluid, we employ a finite-element/fictitious-domain method similar to the distributed Lagrangian multipliers (DLM) method together with a rigid-ring description of the particle. The extensional bi-periodic frame is incorporated by constraint equations with Lagrangian multipliers and is implemented by the mortar element method. In our formulation, the bulk stress is evaluated by simple boundary integrals. Concentrating on 2-D circular disk particles, we present numerical examples of single-particle, two-particle and 100-particleproblems in the extensional bi-periodic frame.We discuss effects of solid fraction and particle configurationon the elongational viscosity of the suspension, in comparisonwith simple shear flow.We found that, at zero strain, the relative elongational viscosityis almost the same as the relative shear viscosityin simple shear for moderately concentrated suspensions. There is a smallincrease in elongational viscosity for large strains,which is related to an anisotropic distribution of the particles.