A 1,2-dioxetane derivative of the formula (I): wherein Ar is an aryl group which may have an alkyl group, an aryl group, a halogen atom, an alkoxyl group, a carboxyl group, a formyl group, an alkyl ester, an aryl ester, an alkylketone, an arylketone or a hetero ring bonded thereto, X is a substituent capable of labeling an organic compound or a biological molecule, or an ester, Y is a hydrogen atom, an acyl group or a group of the formula —Si(R4R5R6) (wherein each of R4, R5 and R6 which are independent of one another, is an alkyl group or an aryl group), Z is an alkyl group, an aryl group, an oxygen atom, a sulfur atom, a carbonyl group, —(CO)—O—, —O—(CO)—, —NH—, —NH—CO—, —CO—NH—, —OSi(R7R8)— (wherein each of R7 and R8 which are independent of each other, is an alkyl group or aryl group) or a group of the formula —(R9R10)SiO— (wherein each of R9 and R10 which are independent of each other, is an alkyl group or an aryl group), each of R1 and R2 is an alkyl group or an aryl group, and R3 is a spacer.