PURPOSE: To obtain the titled compound industrially advantageously without separating an intermediate, etc., by subjecting formaldehyde to addition reaction with an N-substituted thiazolium salt in the presence of a specific amount of a slat, reacting the reaction product with a specified amount of a base.

CONSTITUTION: Formaldehyde is reacted with an N-substituted thiazolium salt (e.g., 3-benzylthiazolium chloride, etc.) in the presence of ≤ chemical equivalent amount based on the thiazolium salt of a base (e.g., triethylamine, etc.) preferably in a solvent such as dioxane, etc. at 60W100°C for 10minW2hr, the reaction product is additionally reacted with a base in a ratio to give ≥ a chemical equivalent to the thiazolium salt preferably at room temp.W60°C for 10minW5hr, to give the desired compound.