PURPOSE: To facilitate measurement of a squareness relation between the one side of a building and a crossing side crossing the one side and to improve workability.

CONSTITUTION: A rectangular rod 16 is provided to a reference rod 10 making contact with one side 2 of a door frame 1. A moving body 17 movable from the tip of the rectangular rod 16 along the axis thereof is provided. First and second blocks 21 and 22 movable forward and backward to and from a crossing side 2 of the door frame 1 are located to the one end part of the rectangular rod 10 and the moving body 7, respectively. A detecting part 30 to detect displacement of each block is provided, the reference rod 10 is brought into contact with the side 2 of the door frame 1 and the rectangular rod 17 is extended along the crossing side 2. The reference rod is brought into contact with the crossing side 2 so that displacement of either the first block 21 or the second block 22 is reduced to zero. This constitution determines the displacement state of squareness of the crossing side 3 with the side 2.