To provide a new method for continuously producing a cyanoisoquinoline in excellent productivity.

Cyanoisoquinoline is produced by subjecting an alkylisoquinoline to a vapor-phase catalytic reaction with ammonia and molecular oxygen in the presence of a catalyst containing vanadium oxide and an oxide of at least one kind of element selected from molybdenum and antimony such as a composition of the formula (1) VaQbOc (Q is at least one element selected from molybdenum and antimony; subscript (a), (b) and (c) are each the atom ratio of vanadium, the atom ratio of at least one kind of element selected from molybdenum and antimony and the atom ratio of oxygen; when (a) is 1, (b) is 2-11 and (c) is a value obtained from the valence of oxygen and the valance of the other element).