For entangled linear monodisperse polymers, uniaxial elongational flow behavior was examined with the primitive chain network (PCN) simulation, which was originally formulated for a network of Gaussian chains bound by sliplinks but was modified in this study to properly take into account the finite extensibility of actual chains. On an increase of the elongational rate (epsilon) over dot from the terminal relaxation frequency 1/tau(d) (at equilibrium) to the Rouse relaxation frequency 1/tau(R), the original and modified simulations gave an indistinguishable steady state elongational viscosity eta(E) that almost scaled as (epsilon) over dot(-1/2). On a further increase of (epsilon) over dot > 1/tau(R), eta(E) obtained from the original PCN simulation diverged to infinity (as noted also for the unentangled Rouse chains). In contrast, eta(E) deduced from the modified simulation increased but did not diverge with increasing (epsilon) over dot > 1/tau(R) (similarly to the behavior of FENE dumbbells). This feature of the modified PCN simulation, i.e., hardening to a finite (nondiverging) level, mimicked the eta(E) data of entangled semidilute solutions, which naturally) reflected the finite extensibility of actual chains. Analysis of the simulation results suggested that the power law behavior (eta(E) similar to (epsilon) over dot(-1/2)) at 1/tau(d) < (epsilon) over dot < 1 tau(R) is related to reduction of the entanglement density and the corresponding reduction of chain tension, while the hardening (upturn of eta(E) at (epsilon) over dot > 1/tau(R)) results from stretch of the chain (eventually approaching full stretch), thus shedding light on the behavior of semidilute solutions. Nevertheless, the modified simulation did not describe the behavior of entangled melts, i.e., eta(E) similar to (epsilon) over dot(-1/2) even at (epsilon) over dot > 1/tau(R). A factor missing in the modified simulation is discussed in an attempt to elucidate, from a molecular point of view, the difference between entangled solutions and melts.