NPP6 as the fluorescent probe which can do high sensitivity and to detect uniquely, the chemical compound useful formula (i) (as for R1 and R2 the hydrogen atom, the alkyl group, or alkoxy group As for R3 and R4 hydrogen atom or halogen atom As for m 0 or 1 As for L carbon count alkylene basis 2 - 5 R5 - N + (R6) (R7) (R8) or - N (R9) (R10) (R the alkyl group) shows6 - R10).NPP6を高感度かつ特異的に検出することができる蛍光プローブとして有用な式(I)の化合物〔R1及びR2は水素原子、アルキル基、又はアルコキシ基;R3及びR4は水素原子又はハロゲン原子;mは0又は1;Lは炭素数2~5のアルキレン基;R5は-N+(R6)(R7)(R8)又は-N(R9)(R10)(R6~R10はアルキル基)を示す〕。