1. A marker detecting system for extracting as a feature a segment containing a distinctive pattern in a background video image not containing a marker in an environment in which detection of the marker is performed, selecting as a unique feature a portion having a frequency equal to or smaller than a predefined value from a frequency distribution of the features, the unique feature does not appear in said background video image, and detecting a marker generated based on said unique feature, comprising: a storage unit for storing the unique feature of said marker; and at least one processor configured to check the unique feature in the video image subjected to detection, which contains said marker in an environment in which detection of a marker is performed, with the unique feature of said marker, and notify detection of the marker when a match of the unique features is found.