In a recent trend that reduction of SF₆ used for electric power equipment is required to avoid global warming, dilution of SF₆ by other gases of less environmental load is considered as a practical solution. Gas mixtures such as SF₆/N₂ and SF₆/c-C₄F₈ have a desirable feature called synergy effect that the critical values of reduced electric field (E/N)_lim are higher than the partial-pressure-weighted values obtained from (E/N)_lim of their components. In order to obtain a guide to seek for new insulating gas mixtures with the synergy effect, we have investigated the synergy mechanism from a viewpoint of collaboration between electron energy loss by collisions and electron attachment. The following mechanisms have been indicated as primary factors of the synergy effect; (i) a large attachment cross section of SF6 lying in a low electron energy region plays the main role of the electron capture; (ii) in order to feed electrons into the low energy region of (i) efficiently, a steep rise of an inelastic collision cross section at its onset energy is required. We have evaluated these factors by a Boltzmann equation analysis, and have synthesized the synergy effect using model gases for demonstration.