To readily and accurately measure a shape and a size of a room wherein tatami mats are laid without troublesome manual operation by a method wherein orthogonal reference lines are stretched on a floor in the room in a condition that the orthogonal reference lines are accurately intersected by using a plurality of right-angled triangles.

Wires 6 forming orthogonal reference lines L are connected so as to be intersected and two right-angled triangles 2 are integrally formed on orthogonal and symmetrical positions starting from an intersection point 1. Each triangle 2 is so constituted that a string material 5 is suspended between one wire 6 and the other wire 6. A length adjustment member 7 is provided to each end section of the wire 6 so that the length of the wire 6 can be adjusted by means of the member 7. Each of the lines L is wound around a winding section of a winding/storing so that a storing condition wherein the line does not become tangled can be maintained and the line L can be smoothly pulled out from the winding section without being tangled when using.