PURPOSE: To easily obtain the subject saccharide or fatty acid in an economically advantageous way using a simple reaction unit by reaction of a lower aldehyde-contg. aqueous solution in the presence of a sulfide ion.

CONSTITUTION: This saccharide or fatty acid is obtained by reaction of (A) an aqueous solution containing a lower aldehyde (e.g. formaldehyde, acetaldehyde) in the presence of (B) a sulfide ion (e.g. sulfur ion liberated at pH2-pH9 from a sulfide such as hydrogen sulfide). For synthesizing a higher fatty acid, it is preferable that (C) a lower fatty acid such as formic acid is used and the concentrations of the components A and B stand e.g. at 10-3-1mol/L, respectively.