An information processor (1) comprises: a history packet transmitter (11) for transmitting via a plurality of ports of a communication processor (2) a plurality of history packets; a request packet transmitter (12) for transmitting a request packet requesting transmission of a return packet; a packet transmission controller (13) for controlling transmission of a request packet; a return packet receiver (14) for receiving a return packet transmitted from a server (3); and a port keeping time detector (15) for detecting the port keeping time of the communication processor (2) based on reception of a return packet by the return packet receiver (14). The server (3) includes: a request packet receiver (31) for receiving the request packet; and a return packet transmitter (32) for transmitting the return packet to the history port of the communication processor (2) where a history packet passed when a request packet receiver (31) received a request packet. This configuration provides an information processing system for detecting the port keeping time of the communication processor.