An acquiring unit acquires information of the residence of one or more second users each having a second social media account related to a first user having a first social media account. An estimating unit estimates the residence of the first user from the information of the residence of the second user. A detecting unit detects a posting area at which the first user has posted on social media. A comparing unit compares the detected posting area with the estimated residence of the first user. A determining unit determines whether the posting area is an ordinary activity area or an extraordinary activity area for the first user based on the result of the comparison.