We found that photomerocyanine isomerized from spiropyran (SP-1822) forms two different kinds of J-aggregation by use of second-harmonic generation and absorption spectroscopy. First, a J aggregate, which has an absorption peak at 608 nm in a monolayer at the air/water interface under high surface pressure : since the molecules in the aggregate align in a certain direction with the same sense, strong second-harmonic light generates in the monolayer. On deposition, the aggregate dissociates and the molecules change into monomers. After a while, the molecules start to form new J aggregates. Promotion of the association by heating and the fact that these are no second-harmonic activities of the aggregates suggest that the molecules cancel each other’s polarity in the second J aggregate.