PURPOSE: To readily obtain the subject compounds useful as a synthetic raw material for various chemicals in high yield by thermally isomerizing a thiocyanic acid ester in the presence of a catalyst such as a quaternary ammonium salt or a phosphonium salt in the absence of a solvent.

CONSTITUTION: A thiocyanic acid ester (e.g. n-propyl thiocyanate) expressed by formula I (R is 1-18C alkyl, aralkyl, cycloalkyl, alkenyl, alkynyl or heterocyclic ring) is thermally isomerized in the presence of a quaternary ammonium salt (e.g. tetraethylammonium bromide) expressed by formula II (R1 to R3 are alkyl; R4 is alkyl or aralkyl; X is halogen) or a phosphonium salt (e.g. tetraethylphosphonium bromide) expressed by formula III as a catalyst in the absence of a solvent to afford the objective isothiocyanic acid esters (e.g. n- propyl isothiocyanate) expressed by formula IV. The above-mentioned method is an epoch-making method for production having high economical efficiency.