1. An update detecting apparatus for detecting an update in a target file which is a file as a detection target of update, comprising: a difference extractor for extracting the difference between text data included in said target file and text data included in a copy file obtained by copying said target file; a divider for dividing said difference into words and generating a word group; and a determining unit for determining that an update is performed in said target file when a word satisfying a predetermined condition is included in said word group.