A map information system includes: an in-vehicle device that executes driving support control based on map information; an external device having external map information used for the driving support control; and an update determination device. The in-vehicle device further executes external update processing that updates first map information being the map information of a first area by using first external map information being the external map information of the first area. The update determination device calculates a change degree being a difference between the first map information and the first external map information, for each point or each area in the first area. The external update processing is prohibited with respect to a section in which the change degree is equal to or less than a threshold. The external update processing is permitted with respect to a section in which the change degree exceeds the threshold.