There is provided a technique which, in the case where the operation has not occurred in a manner as expected by the user, determines from which standpoint the operation was not as expected, and makes a response that is closer to the expectation.An information processing system according to the present invention includes: a reaction section for executing a first process based on a request from a user; an output section for outputting a result of a process; a signal detection section for measuring a signal concerning an event-related potential of electroencephalograms of the user; an analysis section for identifying a positive peak in the event-related potential and determining whether an elapsed time since the result is output and until the peak appears is closer to a first reference time or a second reference time; and a reaction modifying section for determining a second process in accordance with the determination result. This determination result indicates a degree of disappointment of the user, and the second process is to be determined in accordance with the degree. By the reaction section executing the second process, an operation is realized which conforms to the user's expectation even if the operation has not occurred in a manner as expected by the user.