To clarify functionality of depolymerized alginate obtained by hydrolysis of alginate through a heating process at $121^{circ}C$ on gastrointestinal physiology, the effects of a depolymerized alginate on fecal microflora and the intestinal environment were studied in rats. Rats were fed with diets containing 1, 5 and $10\%$ of each depolymerized alginate (HAG-10, HAG-50 and HAG-100) and alginate for 35 days, Among 20 species of authentic intestinal bacteria, Bacteroides ovatus showed the abilities to ferment HAG-10 HAG-50, HAG-100 and alginate, The 1, 5 and $10\%$ of HAG-50 and $1\%$ of alginate diets resulted in the increased ratio of Bifidobacterium and Lactobacillus to total bacteria, while the 5 and $10\%$ alginate diets decreased in feces of rats, significantly (p<0.01). These results suggested that $5\%$ HAG-50 diets contribute to an improvement of intestinal microflora in rats.