PURPOSE: To provide an efficient method for producing aldehydes capable of enhancing recovery efficiency of unreacted olefin and the reaction rate in hydroformylation in a method for recovering unreacted olefin in the liquid reaction mixture of hydroformylation through a gas-liquid contact tower.

CONSTITUTION: This method comprises the following processes: a liquid mixture containing unreacted olefin and aldehyde product contained in a runoff material from a reaction region of hydroformylation is heated; subsequently the liquid mixture is introduced into a gas-liquid contact region to be brought into contact with carbon monoxide and hydrogen in counter-current flow to separate and recover the unreacted olefin from the liquid mixture; and the recovered unreacted olefin is introduced into a reaction region of hydroformylation together with carbon monoxide and hydrogen.