Provided is an industrially advantageous method for producing dialdehyde having a linear dialdehyde and branched dialdehyde production ratio of 80/20-90/10 using a smaller amount of rhodium than in the past. Specifically, provided is a method for producing dialdehyde by reacting linear olefinic compounds, each of which has an ethylenic double bond and an aldehyde group at the end of the molecule, with hydrogen and carbon monoxide in the presence of a rhodium catalyst comprising a bisphosphite shown by general formula (I) (in the formula, R represents a hydrogen atom, a C1-4 alkyl group, or a C1-4 alkoxy group, W represents a C1-20 alkylene group, a C5-18 cycloalkylene group, or a C7-11 alkylene-arylene group) and a rhodium compound, wherein the reaction pressure of a mixed gas comprising hydrogen and carbon monoxide is lowered as the reaction advances.