Provided is a method of producing lavandulal at a high yield by controlling formation of its isomer of lavandulal as a by-product. In a method of producing lavandulal by making an acetal compound represented by the following formula (I) react with 3-methyl-1-butene-3-ol in the presence of an acid catalyst, the method includes: adding, to a liquid mixture comprising an acid catalyst, an acetal compound represented by the following formula (I), and 3-methyl-1-butene-3-ol in at least a portion of an amount to be used (3-methyl-1-butene-3-ol (a)), 3-methyl-1-butene-3-ol in the other portion of the amount to be used (3-methyl-1-butene-3-ol (b)); and maintaining the liquid mixture at a temperature of 110°C or higher: in which Each R in the formula (I) represents an alkyl group.
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