PURPOSE: To draw a horizontal line or straight line having a prescribed gradient on a vertical surface, such as a blackboard, easily and securely by scaling the protraction positions of a protracting ruler according to air-bubble positions of a liquid in a hemispherical transparent tube.

CONSTITUTION: A protracting ruler 1 is provided with a hemispherical transparent tube 3, wherein an air bubble 4 and a liquid are sealed while slanting angle scale of the upper and lower ends 5 and 6 of the ruler 1 is provided along the pipe 3 by indicating a horizontal level by a 0. Consequently, a horizontal line or straight line having a desired slanting angle is drawn easily and securely on the vertical surface of a blackboard, etc., without using a protractor, etc.