A behavior of a group of persons is monitored. Each person is given a radio frequency identification tag. Tag identification information that is unique to the radio frequency identification tag, and reader identification information that is unique to a tag reader, which detects the radio frequency identification tag, are acquired. A position of each of the persons in the group is detected based on the tag identification information and the reader identification information. A position of majority of the persons in the group is regarded as the position of the group. If the position of a person is different from the position of the group, then that person is determined as having parted from the group.