Abstract: The present invention provides a novel process for preparing the compound I containing the compound [I(A)] (vadadustat) and the compound [I(B)] (roxadustat) which are useful as a medicine in an industrially advantageous manner, and a process for preparing the compound I which comprises reacting the compound (a1) or the compound (b1), the compound (2) or salts of the compound (2), with carbon monoxide in the presence of a palladium catalyst, in the presence or absence of a ligand, in the presence of a base, in a solvent to prepare the compound I (wherein structure formulae of the compound (a1), the compound (b1) or the compound (2), and the definitions of groups in the formulae are defined in the Description).