Provided is a technique with which it is possible to promote growth in environments in which Bifidobacterium bacteria or lactic acid bacteria do not grow or do not change in number, or to suppress a decrease in environments in which the bacteria decrease. Provided is an agent for promoting the growth of, and/or suppressing a decrease in, Bifidobacterium bacteria and/or lactic acid bacteria, the agent having as an active ingredient Arthrospira blue-green algae and/or Spirulina blue-green algae. Also provided is an oral composition including Arthrospira blue-green algae and/or Spirulina blue-green algae and at least one type of bacteria selected from the group consisting of Bifidobacterium bacteria and lactic acid bacteria. Additionally provided is a method for promoting the growth of, and/or suppressing a decrease in, Bifidobacterium bacteria and/or lactic acid bacteria, the method having a step that involves the joint presence of at least one type of bacteria selected from the group consisting of Bifidobacterium bacteria and lactic acid bacteria, and Arthrospira blue-green algae and/or Spirulina blue-green algae.
