An image decrypting apparatus derives spectral reflectance of the face of an original on the basis of obtained original image data, built-in light source data and basis function data, and obtains weighted coefficients of each pixel as object color component data. The apparatus stores a file including the basis function data used at the time of obtaining the spectral reflectance as a key file, and stores a file including the object color component data as an encrypted file. From each of the key file and the encrypted file, the original image data cannot be reproduced. By using the key file and the encrypted file in a correct combination, the original image data can be reproduced. In such a manner, the original image data can be protected.