An external standard solution for use in the analysis of amino acid in plasma, containing,(1) at least one amino acid selected from the following components A, at a concentration of 0.0007 M to 0.49 M, and (2) (i) at least one amino acid selected from the following components B, at a concentration of 0.2 to 0.9 times of the lowest-concentration amino acid among the amino acids selected from components A, (ii) at least one amino acid selected from the following components C, at a concentration of 0.1 to 0.4 times of the lowest-concentration amino acid among amino acids selected from the components A, or (iii) at least one amino acid selected from the following components D, at a concentration of 0.05 to 0.2 times of the lowest-concentration amino acid among amino acids selected from the components A; [Components A] valine, glycine, alanine and glutamine [Components B] serine, proline, threonine, taurine, leucine, isoleucine, lysine, histidine, phenylalanine and tyrosine [Components C] asparagine, ornithine, arginine and tryptophan [Components D] glutamic acid, methionine, citrulline and cystine.