Provided is a motion sickness reduction system capable of providing a proper treatment tailored to a motion sickness state of each of two or more users. One aspect of the present disclosure is a motion sickness reduction system including a sensor that senses on-board states of the users aboard a vehicle, two or more communication terminals each assigned to the corresponding user, and an information processor that communicates with the communication terminals. The communication terminals each store motion sickness susceptibility data of the corresponding user to whom each communication terminal is assigned. The information processor performs a sickness level estimation process of estimating a sickness level of each user using the motion sickness susceptibility data and the corresponding on-board state sensed by the sensor, and a reduction treatment process of setting, for each user, a sickness reduction treatment based on the sickness level estimated.