PURPOSE: To obtain 3-formylfuran of high purity which is useful as a synthetic intermediate for perfumes or the like in high yield by using a bromoalkoxybutanediol as a starting substance to effect reaction under specific conditions through simple operations.

CONSTITUTION: In the presence of the first catalyst such as sulfuric acid or the like, 2-bromo-3-alkoxybutane-1,4-diol is allowed to react with a trialkyl orthoformate to prepare 4,5,6,7-tetrahydro-2,5-dialkoxy-5-bromo-1,3-dioxepin. This tetrahydro-1,3-dioxepin is allowed to react with a base to give 4,5- dihydro-2,5-dialkoxy-1,3-dioxepin. The product is refluxed in the presence of silica gel to form a mixture of 2,3-dihydro-3-alkoxy-4-formylfuran with 2,5- dihydro-5-alkoxy-4-formylfuran, then the mixture is refluxed in the presence of the second catalyst such as phosphoric acid to give the objective 3- formylfuran.