Disclosed are novel N-oxyl compounds of the following formula (1). wherein n is an integer of 1 to 18; R1 and R2 are each hydrogen or methyl, but at least one of them is hydrogen; R3, R4, R5 and R6 are each a straight-chain or branched alkyl group; and R7 is hydrogen or (meth)acryloyl. When these compounds are added to vinyl monomers such as &agr;,&bgr;-unsaturated carboxylic acids and esters thereof, they exhibit a satisfactory polymerization-inhibiting effect even at low contents and even at elevated temperatures.