Abstract: The present disclosure provides a technique for introducing an antigen receptor-encoding nucleic acid into a definite site in a nucleic acid that is contained in an immune cell of a target organism. In the technique according to the present disclosure, a vector, which contains an antigen receptor-encoding nucleic acid, and a nuclease are introduced into an immune cell. The nuclease cleaves the vector and the genome sequence of the immune cell. The vector can be configured so that, when cleaved with the nuclease, a sequence, which is an antigen receptor-encoding sequence and which is introduced by microhomology-mediated end-joining (MMEJ) at the nuclease cleavage site of the genome sequence of the immune cell, is formed.