An inspection tool for measuring the true of metal door frames uses a plumb bob. The randomly uneven front surface of the diffuse reflection member diffusely reflects the sound waves, thereby preventing the reflected sound waves from reaching the microphones at the proper phase difference. A cover is seated upon the frame, the cover enclosing the first spool surface. Furthermore, an annular protrusion 43, which protrudes downward, is provided in the boundary region between the annular portions 41, 42. The dialkyl peroxydicarbonates are employed in the aqueous suspension polymerization of halogenated monomers in the form of an organic solution in a water-insoluble organic solvent chosen from conventional chain-regulating agents for halogenated polymers.