Provided is an improved pH low insertion peptide which is obtained by repeating the sequence of an extracellular domain of a pH low inversion peptide existing in the prior art one or more times on the basis of the sequence of the pH low inversion peptide. It is proved, by using a tumor cell cultured in vitro, that the improved low pH insertion peptide is targeted to the surface of the tumor cell. In addition, also provided are a pH low insertion peptide, and a composition constituted by an improved version of the pH low insertion peptide. The composition is used for treating, diagnosing, and marking a tumor. The extracellular domains of the pH low insertion peptide and the improved version thereof are antigenic, and an antibody obtained by immunizing with the extracellular domains can be used for treating a tumor.