To provide a ruler for measuring differences among heights of a plurality of buildings, to solve the problem such that the necessity of much calculation time and frequent miscalculations are caused because each of the heights of the other buildings is subtracted from the height of a standard building, when the heights of the plurality of buildings are measured.

In a ruler for measuring a difference in height, dimensions in units of millimeters are marked in upper and lower positions centering on a scale of 0 on the ruler; a character for "low" is marked in the upper position; a character for "high" is marked in the lower position; protrusions are provided on front parts at both ends thereof; and a non-slip part is attached to a backside thereof.