The interstitial fluid of the skin contains glucose levels comparable to those of blood. Noninvasive glucose sensing by microwaves has great potential to relieve diabetics from the burden of daily blood sampling, but improving the selectivity of this method remains a challenge. This study reports a dielectrically equivalent multilayer skin phantom and provides insight into the criteria for noninvasive glucose sensing by conducting dielectric analysis. The skin phantom was a hydrogel composed of gelatin, glucose, sodium chloride, and water covered by paraffin-impregnated paper. Investigations conducted on a wide range of component concentrations revealed characteristic relative permittivity and dielectric loss determined by the amount of electrolyte and solution that was independent of the amount of glucose. Since the microwave response due to glucose tends to be buried in noise, we developed a flowchart that first identifies the amounts of electrolytes and proteins, which are the major components other than glucose, and then quantifies the remaining glucose content. This noninvasive glucose sensing method would not be limited to the medical healthcare field; it could potentially be used in food manufacturing processes, livestock farming, and plant cultivation management.