PURPOSE: To efficiently obtain the subject compound useful as an agricultural and horticultural germicide, etc., at a low cost by reacting a specific quinoline derivative with an oxyamine derivative.

CONSTITUTION: A 1-alkoxy-4-methoxyimino-2-methyl-3-oxo-1,2,3,4- tetrahydroisoquinoline derivative of formula I (R1 is a 1-4C alkyl) is reacted with an oxyamine of the formula H2NO-R2 {R2 is H or the formula C(R3)(R4)-Ar [R3 and R4 are each H or a 1-4C alkyl; Ar is a (substituted)aryl or a (substituted)heteroaryl]} in the presence or absence of an acid, e.g. sulfonic acids in a polar solvent such as tetrahydrofuran under conditions of 20-120°C, etc., to afford a compound of formula II (e.g. 1-methoxy-4-methoxyimino-2- methyl-3-oxo-1,2,3,4-tetrahydroisoquinoline).