Compounds represented by formula (II): wherein M is a hydrogen atom, sodium, potassium, or lithium; P is a hydrogen atom, an alkyl group, and the like; and the wavy line indicates a cis form, a trans form, or a mixture thereof for the double bond to which it is attached, may be prepared by hydrolyzing a compound represented by formula (I), or a salt thereof: wherein R1 and R2 are the same or different and each is an alkyl group, or R1 and R2 together with the adjacent nitrogen atom may form an aliphatic heterocycle, and P and the wavy line are as defined above, in the presence of alkali metal hydroxide.