We investigated the population size structure of Glehnia littoralis at three beaches, Nishikinohama, Okadaura, and Satoumi-koen Park, in Osaka in 2008 and 2013. There was a correlation between leaf size and the number of seeds produced. Population Size structure was greater at Okadaura than at the other beaches. Therefore, it was estimated that the greatest seed production occurred at Okadaura. Okadaura is located in the Kashii river estuary, and it was believed that seeds could easily be carried to the offshore. We therefore consider that Okadaura is the most important source of seeds among the three beaches.