Area data indicating an encryption area of image data and level data indicating an authority level for the encryption area are received. Upon being triggered by an input password, an encryption key is generated for each received authority level. Check data to be detected in verifying whether or not the encrypted image data in the encryption area is appropriately decrypted through decryption is implanted in the encryption area. Image encryption is performed on the encryption area where the check data is implanted, by using the encryption key corresponding to the authority level designated for the encryption area. In decryption, all decryption keys corresponding to all authority levels lower than or equal to the set authority level are used to decrypt each encryption area and determines whether or not an appropriate decryption has been performed on the basis of the result of the detection of the check data.