A compound represented by the formula (I) (R1 represents an aryl group such as hydroxyphenyl group, R2 represents 2-carboxyphenyl group etc., and R3 and R4 represent —(CH2)p—N(R5)(R6) (R5 and R6 represent —(CH2)n—COOH), a salt thereof, or an ester thereof, which is useful as a reagent for highly sensitive measurement of reactive oxygen such as hydroxy radical present at a trace amount in living bodies or tissues by a bioimaging technique over a long period of time.