The present invention provides a process for producing an .alpha.,.alpha.-bis(hydroxymethyl)alkanal represented by Formula (II): ##STR00001## (wherein R represents an alkyl group, a cycloalkyl group, or an aryl group) which comprises reacting an aldehyde represented by Formula (I): R--CH.sub.2--CHO (I) (wherein R has the same meaning as defined above) with formaldehyde in the presence of a basic catalyst and a phase-transfer catalyst.