PURPOSE: To omit welding work, decrease the cost of manufacture and to facilitate collection by tightening and fixing a plurality of crushable pipes and connection plates staying between upper and lower plates, in the direction of an axial line.

CONSTITUTION: A plurality of through holes 2a, 5a, 3a are provided with their axial lines vertically aligning to each other in a part where crushable pipes 4 in an upper plate 2, connection plate 5 and a lower plate 3 are fixed. A tightening rod 11 is guided to penetrate through inside a plurality of the crushable pipes 4, and upper and lower end parts of this rod 11 are screwed to upper and lower cover units 6, 10. In this way, the crushable pipes 4 and the connection plates 5 staying between the upper and lower plates 2, 3, are tightened and fixed in the direction of the axial line. Thus by omitting welding work, the cost of manufacture is decreased, and collection is facilitated.