The oil-filled electrical appliance in claim 7, wherein said aromatic monoolefins are the compounds represented by the following general formulae (IV) to (VI): ##STR16## wherein any one of R1, R2, R3 and R4 is an aryl group or an aralkyl group and the others are a hydrogen atom or an alkyl group, n is an integer from 0 to 3, when R4 is an aryl group or an aralkyl group, n is 1, the symbol .....