It is an objective of the present invention to provide tRNA that has CUA or CCCG as an anticodon and is aminoacylated with an unnatural amino acid, such tRNA being capable of efficiently introducing an unnatural amino acid into a protein without competing with a termination factor. Such tRNA is a mutant tRNA for tryptophan which has G at the 5′ end, C as a base pairing with the G at the 5′ end, and A as a base next to the C on the 3′ side, such tRNA being a mutant tRNA which pairs with a stop codon and has CUA as an anticodon or a mutant tRNA which pairs with a stop codon or a 4-base codon has CUA or CCCG as an anticodon, into which a single base has been inserted just before the CCA sequence at the 3′ end.