Single molecule conductance of a series of porphyrin derivatives, whose porphyrin units are connected through a phenylene bridge that have one or two thiolate end groups, was measured by a scanning tunneling microscopic break junction technique. The observed conductance values were analyzed by partial transmission probabilities (T_i) for the molecular parts using the equation G = G₀×T₁×T₂×T₃×... Unique combination of T_i was obtained, which indicates the validity of this analysis. Interestingly, the free-base porphyrin ring interacting with the gold electrode showed a greater transmission probability T_por-Au than that of the bridge between thiolate and the gold electrode (T_S-Au). This indicates that the porphyrin free base work as a good anchoring group to gold electrode.