A map data production system includes a workstation, a keyboard, a mouse, a data storage and a display. The data storage stores road map data. A section of a road on a map read from the data storage and displayed on a display. An access road, grade data of which will be added or altered, is specified by clicking its start and end points on the map with the mouse. An elevation value of the road in the specified section is entered at a data entry screen display on the display by a user via the keyboard. A grade value of the road in the specified section is calculated from the entered elevation value and map data stored in the data storage. The calculated grade value is added to the road map data and stored in the data storage.