A method for downloading software from a host device to an electronic device through a communication line, which, even when the download is interrupted, can simplify the procedure to restart the download while maintaining security. In the method, a certificate of authenticity data, which the card reader has obtained from the HOST computer, is stored in the non volatile memory. The download of the software from the HOST computer to the card reader is executed. The verification of authenticity data is obtained by calculation with respect to the downloaded software. This verification of authenticity data is then compared with the certificate of authenticity data obtained from the HOST computer, and the downloaded software is run when the certificate of authenticity data matches the verification of authenticity data.