Investigating sweat component concentrations is important for managing electrolyte balance and water content in the body, monitoring hydration and heat stress, and preventing dehydration. Predicting rehydration timing for regulating body temperature and preventing dehydration is particularly challenging because it depends on the individual’s health and exercise status, which affects the amount and composition of sweat. Consequently, there is a demand for a simple analytical method using a portable device for practical applications. In this study, sodium and potassium concentrations were evaluated in aqueous solutions using multi-wavelength NIR spectroscopy. The number of wavelengths necessary for accurate analysis was also verified using multiple regression analysis. The concentrations of aqueous sodium and potassium electrolytes were evaluated by multiple regression analysis using the differential absorbance of the samples obtained using a NIR spectrometer. By using four and five wavelengths for each of the two electrolytes, the accuracy of the correlation coefficient between the optically evaluated concentration and the concentration measured using a commercial ion meter exceeded 0.9995. A multi-wavelength NIR spectroscopic method was developed for the optical evaluation of electrolyte concentrations in aqueous solution. The concentration evaluation was achieved using two wavelengths, leading to the development of a simple device for measuring sweat components.