A measurement technique of thermodynamic variables with high sensitivity is strongly demanded for analyses of highly rarefied gas flows. REMPI (Resonantly Enhanced Multiphoton Ionization) is a powerful optical tool because of its high sensitivity even in highly rarefied gas flows and ability to measure nonequilibrium among internal (translational, vibrational, and rotational) energy. In this study, to investigate fundamental properties of REMPI signal, the experimental apparatus of 2R N2-REMPI is constructed and the REMPI spectra are measured along the center line of a free molecular jet. A method of Boltzmann plot using the spectral lines of both O and P branches is proposed, determining electronic transition dipole moments in Hönl-London factors experimentally from the relative line strength of O and P branches. Then an effect of an experimental noise on a rotational temperature measured by the Boltzmann plot becomes smaller because the number of the lines used for the plot increases. By using the factors, simulated spectra agree well with experimental ones, suggesting feasibility of rotational temperature measurement by a spectral fitting.