An NF-kappaB inhibitor which can be easily obtained and consumed in daily dietary life and can inhibit transcription activation by NF-kappaB is provided. When osteoblasts in which NF-kappaB is activated by TNF-alpha is cultured in a medium containing Sargassum horneri or a processed product thereof, for example, a Sargassum horneri water extract of molecular weight up to 3,000, transcription activation by NF-kappaB can be inhibited. In addition, when osteoclast precursor cells in which NF-kappaB is activated by RANKL is cultured in a medium containing Sargassum horneri or a processed product, for example, a Sargassum horneri water extract thereof of molecular weight up to 3,000, transcription activation by NF-kappaB can be inhibited.
