  We present an evaluation of box diagram for the elastic $ep$ scattering with
proton in the intermediate state. Using analytic properties of the proton form
factors we express the amplitude via twofold integral, which involves the form
factors in the space-like region only. Therefore experimentally measured form
factors can be used in the calculations directly. The numerical calculation is
done with the form factors extracted by Rosenbluth, as well as by polarization
transfer methods. The dependence of the results on the form factor choice is
small for $Q^2 < 6 GeV^2$, but becomes sizable at higher $Q^2$.
