To realize sustainable societies, the production of organic compounds not based on fossil resources should be developed. Thus, C1 chemistry, utilizing one-carbon compounds as starting materials, has been of increasing importance. In particular, the formose reaction is promising because the reaction produces sugars (monosaccharides) from formaldehyde under basic conditions. On the other hand, since microwave (MW) induces the rotational motion of molecules, MW irradiation often improves the selectivity and efficiency of reactions. In this study, the formose reaction under MW irradiation was thus investigated under various conditions. The formose reaction proceeded very fast using 1.0 mol per kg formaldehyde and 55 mmol per kg calcium hydroxide (Ca(OH)₂) as a catalyst at a high set temperature (150 °C) for a short time (1 min) to form preferentially specific hexose and heptose. The major products were isolated by thin layer chromatography and characterized by mass spectroscopy and NMR. These characterization data elucidated that the hexose and heptose were 2-hydroxymethyl-1,2,4,5-tetrahydroxy-3-pentanone (C6*) and 2,4-bis(hydroxymethyl)-1,2,4,5-tetrahydroxy-3-pentanone (C7*), respectively. On the basis of these observations, as well as density functional theory calculations, a plausible reaction pathway was also discussed; once 1,3-dihydroxyacetone is formed, consecutive aldol reactions favorably occur to form C6* and C7*.