An input control unit that receives first content data from a content delivery apparatus via a communication unit, converts the first content data into first image data before causing a storage unit to store the first image data, receives second content data generated by a portion or all of the first content data being updated from the content delivery apparatus via the communication unit, and converts the second content data into second image data before causing the storage unit to store the second image data, a center of gravity candidate region extraction unit that calculates difference values of color information between corresponding pixels of the first image data and the second image data stored in the storage unit and extracts a center of gravity candidate region, which is a region constituted by pixels the difference values of which exceed a threshold value stored in the storage unit.