To provide a display device which enables to visually perform a work at an identical altitude, because a contour line tillage method is the best method to prevent the outflow of soil and solve a problem that a conventional reasonable method for tilling a farm field along one side of a section is a main method but causes the washing-away of soil and the outflow of surface soil, when it rains to form water flows.

The farm work method comprises calculating contour lines as lines near to straight lines as much as possible on the basis of preliminarily made farm field height map data, displaying at least the lines and the present position on a display on the consideration of various conditions, and working along the lines of the display to become a contour line tillage, thus preventing the outflow of the soil.