PROBLEM TO BE SOLVED: To provide a novel fluorescent probe capable of detecting a biomarker enzyme activity having high specificity for breast cancer with high sensitivity and high selectivity and detecting as a response of red fluorescence of longer wavelength. A fluorescent probe for detection of puromycin-sensitive aminopeptidase (PSA) containing a compound represented by formula (I) or a salt thereof: [wherein A, B, X and Rn are specific groups. ] [Selection diagram] Figure 1【課題】乳がんに特異性の高いバイオマーカー酵素活性を高感度・高選択的に検出できるとともに、より長波長の赤色蛍光の応答として検出することができる新規蛍光プローブの提供。【解決手段】式（Ｉ）で表される化合物又はその塩を含む、ピューロマイシン感受性アミノペプチダーゼ（ＰＳＡ）検出用蛍光プローブ：〔式中、Ａ、Ｂ、Ｘ、Ｒｎは特定の基。〕【選択図】図１
