To develop a method to simultaneously estimate the properties of salt (sodium chloride) products, namely, moisture content, mean particle size, and magnesium content, we conducted a basic test and a demonstration test. In the basic test, the relationships between these properties and the diffuse reflectance of salt products were examined on the basis of infrared diffuse reflectance of spectra recorded using a spectrophotometer. At all wavelengths, the diffuse reflectance of salt was found to change with changes in each property; and the degree to which each property contributes to changes in the diffuse reflectance depends upon the wavelength. Equations to predict the value of each property from the diffuse reflectance of multiple wavelengths were created from the relationships between each property and the spectrum data. Next, to create the optimum combination for simultaneous estimation, we selected the seven wavelengths that best average the prediction accuracies. In the demonstration test, the seven selected wavelengths were applied to a commercially available infrared analyzer. The effects of impurity content (other than magnesium) or any deviation in the mean particle size on the absorbance were found to be insignificant within the quality range of the manufactured salt product. Analytical curves that predict each property from the absorbance at the seven wavelengths were also created. These analytical curves showed good prediction accuracies.