Interfacial ion-association adsorption and aggregation of a water-soluble porphyrin, tetrakis(4-sulfonatephenyl)-porphyrin (TPPS) diacid, which was promoted by a cationic cetyltrimethylammonium ion (CTA+), was studied by second harmonic generation (SHG) spectroscopy. Comparing the interfacial SH spectrum with the transmission absorption spectrum of TPPS in the aqueous solution elucidated the aggregation behavior of TPPS at the heptane/water interface. The time-dependent SHG spectra for TPPS aggregation and the interfacial tension lowering in the presence of CTA+ were discussed on the basis of an electrostatic adsorption model. Then, it was suggested that TPPS diacid was highly concentrated by the ion-association with CTA+ at the interface, which was the intermediate state before the final aggregated state.