1. A map data processing device, comprising: a map database that has first map data of a road on which a vehicle travels; a map data computing module configured to compute second map data of the road based on a surrounding environment of the vehicle and a traveling state of the vehicle; an update data computing module configured to compute update data to update the map database, based on a reliability of the second map data, the reliability being a numerical value that is set based on at least stability of a recognition state of the surrounding environment by one or more cameras equipped on the vehicle, stability of vehicle behavior detected by one or more sensors equipped on the vehicle, and stability of reception state of GPS signals received by the vehicle; an update determining module configured to: compare the first map data with the update data; determine an error between the first map data and the update data; and determine, by an update determination, to update the first map data with the update data, wherein the determination to update the first map data with the update data is based on at least a multiplication product of the reliability and a frequency with which the error is greater than or equal to a first determination value; and a data processing module configured to update the map database with the update data based on at least a result of the update determination.