A method for preparing an aldehyde including forming a reaction product including an aldehyde by reacting an olefin-based compound with a synthetic gas in a hydroformylation reactor in the presence of a hydroformylation catalyst; introducing the reaction product including the aldehyde to a vaporizer; separating low-boiling point components of the reaction product from an upper part of a vaporizer catch pot included in the vaporizer; separating high-boiling point components of the reaction product from a lower part of the vaporizer catch pot; and recirculating at least a portion of the low-boiling point components separated from an upper part of the vaporizer catch pot back to the vaporizer.