|| Secretory immunoglobulin A (SIgA) is predominant antibody secreted into the gut relating to maintain gut homeostasis. In the present study, we focused on the SIgA coating of gut bacteria as a mucosal immune response affecting the gut bacteria following a high-fat diet (HFD). This study newly showed that the level of SIgA coating of gut bacteria was significantly decreased in HFD-fed mice than normal-fat diet (NFD)-fed mice. Furthermore, we found that the relative abundances of gut bacteria were significantly negatively or positively correlated with the level of SIgA coating of gut bacteria in NFD-fed and HFD-fed mice. Our observations suggest that a decrease in the level of SIgA coating of the gut bacteria through a HFD might relate to HFD-induced changes in bacterial composition.