A 5-bromo-1-pentanal compound of the formula (I): ##STR1## wherein R.sup.1 and R.sup.2 respectively represent, independently from each other, a member selected from the group consisting of a hydrogen atom and alkyl radicals having 1 to 7 carbon atoms, or an acetal derivative of said 5-bromo-1-pentanal compound, is produced by subjecting a starting compound selected from the group consisting of 4-pentenal compounds of the formula (II): ##STR2## wherein R.sup.1 and R.sup.2 are as defined above, and acetal derivatives of the above-mentioned compounds, to a free radical addition reaction with hydrogen bromide in a liquid medium at a temperature of 10.degree. C. to 60.degree. C.; in the presence of a free radical initiator or by irradiating ultraviolet rays to the reaction mixture, and by collecting the resultant reaction product from the reaction mixture.