A ruler having an arch member on an upper surface of a ruler body for allowing the ruler body to closely contact a target object by being pressed from above. The ruler has a pair of parallel standing walls facing each other on the upper surface of the ruler body in a longitudinal direction, and the arch member is fitted along the longitudinal direction of the standing walls. In addition, bending pieces which bend inward towards each other are formed on upper end edges of the standing walls, projections which are positioned below the bending pieces project on both of outer side surfaces of the arch member, and the projections are guided by the bending pieces and are inserted between the standing walls.