To provide an effective and simple process for producing 4-fluoroisoquinoline-5-sulfonylhalide or a salt thereof, and a simple method for separating for purification of the product from a by-produced position isomer thereof. The process for producing 4-fluoroisoquinoline-5-sulfonyl halide or a salt thereof, characterized in that the process includes reacting 4-fluoroisoquinoline or a salt thereof with sulfuric anhydride in the presence or absence of sulfuric acid, to thereby form 4-fluoroisoquinoline-5-sulfonic acid or a salt thereof, and, subsequently, reacting the formed sulfonic acid compound with a halogenating reagent.