5-Methyl-4,5,6,7-tetrahydrothiazolo[5,4-c]pyridine-2-carboxylic acid of formula (5) or a salt thereof, [F14] ##STR00001## is prepared by reacting a compound of formula (6) or a salt thereof, ##STR00002## with a trihalogenoacetyl halide in the presence of a base, followed by hydrolysis.