The present invention provides a peptide that is easier to produce and process than conventionally known prothymosin α or its peptide, and has an activity equal to or higher than that. [MEANS FOR SOLVING PROBLEMS] The present invention improves a blood organ barrier disorder, comprising as an active ingredient a peptide comprising the amino acid sequence represented by SEQ ID NO: 2 or SEQ ID NO: 3 or a peptide having a function substantially equivalent thereto. The present invention provides an agent, a therapeutic agent for a disease associated with a blood organ barrier disorder or an ischemic disease, and a neuronal cell death inhibitor. [Selection] Figure 1A【課題】従来公知のプロサイモシンα又はそのペプチドよりも製造・加工が容易であり，それらと同等又はそれ以上の活性を有するペプチド提供すること。【解決手段】本発明は，配列番号２又は配列番号３で表されるアミノ酸配列からなるペプチドまたはこれと実質的に同等の機能を有するペプチドを有効成分として含有してなる，血液臓器関門障害改善剤，血液臓器関門障害を伴う疾患又は虚血性疾患の治療剤，神経細胞死抑制剤を提供する。【選択図】 図１Ａ
