[Problem] To provide a novel fluorescent probe which makes it possible to detect a peptidase activity that is highly developed in a cancer cell or the like as the response of a red fluorescence having a longer wavelength, and which has excellent tissue permeability. [Solution] A compound represented by formula (I) or a salt thereof :[in the formula, A represents a ring structure selected from the group consisting of a thiophene ring, a cyclopentene ring, a cyclopentadiene ring and a furan ring； X represents a C0-C3 alkylene group； Y represents O, S, C(=O)O, or NH； Z represents O, C(Ra)(Rb), Si(Ra)(Rb), Ge(Ra)(Rb), Sn(Ra)(Rb), Se, P(Rc) or P(Rc)(=O) (wherein Ra and Rb independently represent a hydrogen atom or an alkyl group； and Rc represents a hydrogen atom, an alkyl group or an aryl group)； R1 and R2 independently represent one to three same or different substituents which are independently selected from the group consisting of a hydrogen atom, a hydroxyl group, a halogen atom, and an alkyl group, a sulfo group, a carboxyl group, an ester group, an amide group and an azide group each of which may be substituted； R3 represents an acyl residue derived from an amino acid (wherein the acyl residue is a residue produced by removing an OH group from a carboxyl group in an amino acid)； and R4 and R5 independently represent a hydrogen atom or an alkyl group (wherein, when R4 or R5 is an alkyl group, the R4 or R5 may form, in conjunction with R2, a ring structure containing a nitrogen atom to which R4 and R5are bonded)].Le problème décrit par la présente invention est de fournir une nouvelle sonde fluorescente qui permet de détecter une activité peptidase qui est extrêmement développée dans une cellule cancéreuse ou similaire en réponse à une fluorescence rouge ayant une longueur d'onde plus longue, et qui présente une excellente perméabilité tissulaire. La solution selon l'invention porte sur un composé représenté par la formule (I) ou un sel de celui-ci : [dans la formul
