PROBLEM TO BE SOLVED: To provide a novel coumarin compound which has excellent hydrolytic stability, excellent copolymerizability with a styrene compound and good water dispersibility, and is useful as a diagnostic marker.SOLUTION: The coumarin compound is represented by the general formula (1) in the figure. (In the formula, Rand Reach independently represent a C1-5 alkyl group, or a propylene group linked with carbon at positions 6 and 8 of the coumarin skeleton to form a cyclic structure Rrepresents a hydrogen atom, a C1-5 alkyl group, a trifluoromethyl group or a cyano group Rrepresents a hydrogen atom or a C1-5 alkyl group Rrepresents a direct bond or a C1-3 alkylene group and X represents a sulfur atom, an oxygen atom, an NH group or an NCHgroup.)
