PURPOSE:To enable a direct measurement of a spring force at a spring section of a support lattice for a short time by forming a plurality of split cuts at the bottom of a tube provided on one side of an elastic plate member along the member. CONSTITUTION:In a deformed bar 22, one side of a square tube section 25 formed at the upper part thereof is extended downward to form a plate-shaped section (plate member) 26. A diameter-expanded tube 23 has a large-diameter section 30 at the upper part thereof and the upper part thereof is inserted into a hole of the square tubular section 25 while the large diameter section 30 is fixed on the plate-shaped section 26. The lower half of the diameter-expanded tube 23 is equally divided into two with a plurality of vertical split cuts formed at an interval in the circumferential direction. This enables a direct measurement of a spring force at a sprint section of a support lattice for a short time.