To lighten the processing load of embedding a digital watermark in content data, a preliminary process (embedding strength map generating process) run by a preprocessing section and the main process (digital watermark embedding process) run by a man processing section are run mutually independently. Specifically, the preliminary process of the preprocessing section is run each time new content data is stored to the content data database, and the main process of the main processing section is run when a content delivery request is received from a receiving terminal (client).