In a process for producing an aldehyde by hydroformylating allyl alcohol with carbon monoxide and hydrogen gas in the presence of a catalyst comprising a rhodium compound and a tertiary phosphine in an organic solvent and separating the resulting aldehyde with water extraction from the reaction mixture, the catalyst is maintained and recovered in the activity by treating the catalyst-containing solution in the organic solvent with a gaseous mixture of carbon monoxide and hydrogen gas after the water extraction step.