A method for producing aldehydes, which comprises reacting an olefinic unsaturated compound with carbon monoxide and hydrogen for hydroformylation in the presence of a rhodium-phosphite complex catalyst to obtain a reaction product solution containing the rhodium-phosphite complex catalyst and an aldehyde product, and separating from the reaction product solution at least one component selected from the group consisting of carbon monoxide, hydrogen, an unreacted olefinic unsaturated compound, the aldehyde product, a solvent, a medium-boiling by-product and a high-boiling by-product by a separating operation, wherein at least one separating operation is carried out substantially in the absence of water, and the temperature and the residence time in the separating operation are selected to be within such ranges that value P calculated from the following formula (1) would be at most 1: P=5.0.times.10.sup.3 .times.exp�-5000/(T.sub.1 +273)!.times..theta.T.sub.1(1) where T.sub.1 is the maximum temperature (.degree.C.) in the separating operation, and .theta.T.sub.1 is the residence time (minutes) of the liquid in the separating operation.