[Problems to be Solved] It is an object of the present invention to provide an agent and a method for maintaining and promoting the stability of a membrane protein, and in particular, a transporter (for example, an ABC transporter) on a cell membrane.[Solution] The present invention provides: a method of treating a cell with carboxylic acid including 4-phenylbutyric acid or a derivative thereof to suppress the decomposition of a protein existing on a cell membrane, so as to maintain and promote the stability of the protein on the membrane； and an agent for stabilizing a membrane protein, which comprises carboxylic acid including 4-phenylbutyric acid or a derivative thereof
