PURPOSE: To measure the interior angle of an L-shaped sectional object to be measured simply and accurately.

CONSTITUTION: Slide caliper 23 is arranged onto the lower horizontal surface section 22 of a rectangular L-shaped sectional measuring reference base 21, and the main scale 24 of the slide caliper 23 is fixed at a right angle to the vertical surface section 25 of the measuring reference base 21. The vernier 27 of the slide caliper 23 is made to abut on the internal side face of the sharpness section of an external angle member 12, and the projecting size, clearance size D from the sharpness section of the measuring reference base 21 to the internal side face of the sharpness section of the external angle member 21, of the vernier 27 is read with scales of the slide caliper 23. A sliding plate 31 is mounted slidably on the vertical surface section 25 of the measuring reference base 21 in the vertical direction, the sliding plate 31 is made to abut on the upper end of the external angle member 12, and height size H from the lower end face of the measuring reference base 21 to the upper end of the external angle member 12 is measured with the gauge of the vertical surface section 25. The interior angle θof the external angle member 12 is obtained from the relationship of formula θ=90°- tan-1(D/H) on the basis of these measured values D, H.