PURPOSE: To obtain the title compound useful as an intermediate for cephalosporin antibiotic substances industrially and advantageously, by reacting a thiophene aldehyde with a hypohalite.

CONSTITUTION: A thiophene aldehyde expressed by formulaI(R is H, halogen or lower alkyl) is reacted with a hypohalite, e.g. sodium hypochlorite, in an alkaline aqueous medium, e.g. NaOH, to give a thiophenecarboxylic acid expressed by formula II. The reaction is carried out by using 5W50mol%, expressed in terms of the available halogen based on the aldehyde, hypohalite at 50W100°C, and reacting the hypohalite with the resultant reaction solution at 50W100°C. If the reaction system is heated rapidly, the reaction is initiated violently to cause the decomposition of the hypohalite preferentially. Therefore, the necessary amount of the hypohalite is partially used to advance the reaction partially, and the rest is then added to the resultant reaction solution.