An information processing system includes an operation terminal and an input/output device connected to each other. The operation terminal includes a notification unit configured to send a notification based on information from the input/output device indicating that communication is possible with the input/output device. The input/output device includes an imaging unit, a detection unit for detecting the operation terminal capable of communicating with the input/output device, a transmission unit for to transmitting the information indicating that communication is possible with the input/output device, to the detected operation terminal, a determination unit for determining whether the imaging unit has taken an image of a predetermined action after the information has been sent, and an authentication unit for acquiring terminal information from the detected operation terminal, and performing authentication based on the terminal information, when the image of the predetermined action is determined to have been taken by the determination unit.