An image transmission system comprises: one or a plurality of image pickup apparatus each having a unique identifying number and having an encrypting function for encrypting a picked-up image for transmission to a network; a key generating apparatus for generating, for each image pickup apparatus, an encryption key for encrypting the image and a decryption key for decrypting the encrypted image; a memory card for recording the decryption key and the identifying number of the image pickup apparatus in association with each other; a viewing apparatus connected with the memory card and having a decrypting function for decrypting the encrypted image using the decryption key, for viewing the image transmitted via the network by the image pickup apparatus; and an authenticating server for authenticating the image pickup apparatus accessible from the viewing apparatus.