A 11-hydroxy-11-(3-dimethylaminopropyl)-6,11-dihydrodibenzo[b,e]oxepine 2-acetic acid ester represented by formula [I] (wherein R1, R2, and R3 each independently represents C1-4 alkyl) or an acid addition salt thereof is heated in a solvent in the presence of an acid and at least one compound selected from carboxylic acid anhydrides each having 3 to 8 carbon atoms in total, C2-6 carboxylic acid halides, sulfur acid halides, and phosphorus halides. Thus, (Z)-11-(3-dimethylaminopropylidene)-6,11-dihydrodibenzo[b,e]oxepine 2-acetate, which is represented by formula [II], or an acid addition salt thereof can be industrially advantageously produced.
