1. A monitoring method comprising: acquiring tag identification information that is unique to a radio frequency identification tag carried by a person, and reader identification information that is unique to a tag reader capable of detecting the radio frequency identification tag in a predetermined area; acquiring attribute information that is information about the person carrying the radio frequency identification tag based on the tag identification information acquired; acquiring area information that is information about the predetermined area based on the reader identification information; and determining whether the person carrying the radio frequency identification tag is allowed to enter the predetermined area based on the attribute information and the area information.