Attention is paid on the phenomenon that substantially no direct sun light or no reflected light from surrounding objects is applied, to a vehicle bottom side and tire travel surfaces and the vehicle bottom side and the tire travel surfaces have lower luminance values than all the other portions including the shades of trees or buildings along the road in an on-vehicle camera image. A computer processor loads a first value from a first memory address, and records both the first value and the second value in a table or queue. In order to achieve the object, the etchant has fluoric acid, periodic acid and sulfuric acid wherein the total weight ratio of the fluoric acid and periodic acid is 0.