In a monitoring system, a first device among devices includes a generating unit that generates third data indicating a string of an identical section when strings each indicated in first data and second data acquired through communications with the devices are identical to each other by a predetermined proportion or more, and a first transmitting unit that transmits the third data generated by the generating unit and information for identifying the third data to a second device included in the devices. The second device includes a second transmitting unit that transmits, when a string indicated in fourth data to be transmitted to the first device includes the string indicated in the third data, fifth data indicating a string obtained by excluding the string indicated in the third data from the string indicated in the fourth data, and the information for identifying the third data to the first device.