A process for preparing an .alpha.-phenylpropionic acid derivative wherein an .alpha.-phenylethyl alcohol derivative is reacted with carbon monoxide in the presence of a catalyst to give an .alpha.-phenylpropionic acid derivative, which comprises carrying out the reaction in either one of the following manners (I), (II) and (III):(i) effecting said reaction in the presence of a rhodium catalyst, as said catalyst, together with an iodine compound in such an amount as to give a Rh to I ratio on an atomic basis of 1:0.5 to 1:6; adjusting the concentration of water in the reaction mixture to a level of 2 mol/l or below; and employing a reaction temperature of 130.degree. C. or below,(II) employing a mixture of a hydrocarbon and an oxygen-containing organic compound as a solvent, or(III) effecting said reaction in the presence of a rhodium catalyst, as said catalyst, and an iodine compound; adding water optionally together with an oxygen-containing organic compound as an extraction solvent to the resulting reaction mixture to thereby separate the same into two layers, i.e., a layer containing the reaction product and an aqueous layer; and repeatedly using the rhodium catalyst contained in said aqueous layer.