A process for preparing biotin or its ester which comprises the steps of (a) reacting the compound of the formula: <CHEM> with a compound of the formula: (C6H5)3P<+>(CH2)4COOR1.X<-> (II) wherein R1 is a hydrogen atom or a C1-C4 alkyl group and X is a halogen atom in the presence of a base to give a compound of the formula: <CHEM> p
