An object of the present invention is to provide means for efficiently incorporating substances such as lactic acid bacteria, which are useful for intestinal immune induction, into the intestinal tract. According to the present invention, the following are provided: an agent for promoting substance incorporation in the intestinal tract which has a surface layer protein (S-layer protein) having peptide motifs found in common among lactic acid bacteria of the genus Lactobacillus a complex of the agent for promoting substance incorporation in the intestinal tract and a substance to be incorporated into the intestinal tract a beverage or food product or a pharmaceutical product having the complex and a method for screening for a lactic acid bacterium having a high level of ability to be transferred into the intestinal tract, by determining the expression level of an S-layer protein in the surface layers of bacterial cells of a test lactic acid bacterium.
