PURPOSE: To industrially advantageously obtain a naproxen (anti-inflammatory agent) by selectively and safely dehalogenating at the 5-position of a naphthalene nucleus with a ketal of an inexpensive intermediate by a simple operation without using a dangerous material such as hydrogen.

CONSTITUTION: The objective compound expressed by formula III is selectively obtained by treating a compound expressed by formula I (each of X and X1 represents Br or Cl, R represents an alkyl, phenyl, benzyl, a 2-5C alkylene with two Rs corporated, (ROCOR of a cyclic structure, which can be substituted by an alkyl) or formula II with two Rs incorporated (R1 represents OH, an alkoxyl or an amine group)) with a dehalagenating agent of one or more kinds selected from HS, an aliphatic thiol, preferably HS, RSH, (R represents methyl, ethyl, n-propyl, n-butyl or tert-butyl) thioglycolic acid and 1,2-ethane dithiol, particularly tert-butyl mercaptane in an inert solvent at an acidic pH. In such a case, the compound expressed by formula I obtained after brominating a compound expressed by formula IV can be dehalogenated in the same reaction system without isolating.