Disclosed is a novel process which has low facility load, requires a reduced amount of energy, is highly safe, and enables to produce a monosaccharide such as glucose and xylose by decomposing a cellulose-based substance under conditions where a substance used for decomposing the cellulose-based substance (e.g., a solvent, a catalyst) can be separated, collected and re-used readily. Also disclosed is a novel process for producing ethanol from the monosaccharide through ethanol fermentation. Specifically disclosed is a process for producing a monosaccharide, which comprises disaggregating/finely pulverizing a cellulose-based substance, mixing the resulting material with a phosphate solution to disperse the resulting material in the phosphate solution, and decomposing cellulose in the solution at a temperature of 100.degree. C. or lower while irradiating with ultraviolet ray optionally in the presence of titanium dioxide.