1. A map data processing device for a vehicle, the map data processing device comprising: a map database that has first map data of a road on which the vehicle travels; a map data computing module that computes second map data of the road on the basis of a surrounding environment of the vehicle and a traveling state of the vehicle; an update data computing module that computes update data for updating the map database, on the basis of a reliability of the second map data; an update determining module that compares the first map data with the update data, and determines, when an error between the first map data and the update data is greater than or equal to a determination value, whether to update the first map data with the update data, on the basis of the reliability and a frequency of discrepancy in which the error is greater than or equal to the determination value; and a data processing module that executes a processing of updating the map database in accordance with a determination result by the update determining module.