The hydroformylation of 3-methyl-3-buten-1-ol with hydrogen and carbon monoxide, when carried out under relatively mild conditions, gives 2-hydroxy-­4-methyltetrahydropyran (MHP) in high rate of reaction and in high selectivity even in a very low rhodium catalyst concentration. MHP is useful as a starting material for the production of 3-methyl-1,5-pentanediol and β-methyl-δ-valerolactone which are in turn usable as raw materials for producing polyesters and polyurethanes. The process according to the invention can give such MHP advantageously from the industrial viewpoint and has very great industrial utility.