We have monitored the status of Black-tailed Gulls Larus crassirostris breeding on Tobishima Island since 2004. On Yurijima Islet, we estimated 2,034 nests in 1 June 2014 and observed a single cat Felis catus in the breeding colony and the carcasses of 14 adult Black-tailed gulls that probably were killed by a cat. We also found feathers of adult Black-tailed Gulls in a single cat feces collected on the mainland of Tobishima. Approximately 1,500 nests were estimated on Tateiwa Islet in 2009, but no nests were found in May 2014. To conserve the breeding sites of Black-tailed Gulls at Tobishima, immediate control management of the feral cat population is essential.