PURPOSE: To facilitate mud drainage and exchange of a pivot shaft by providing a bracket part on a tank side bracket, and making the space between an oil tank and the tank side bracket larger than the length of the pivot shaft.

CONSTITUTION: A boom bracket 11b on an oil tank side 5 is provided with a bracket part 11b' allowing the pass of the pivot shaft 10 of a boom cylinder C1. The space L between a cylinder bracket 12 and the tank 5 is set larger than the length of the pivot shaft 10. Mud drainage is enhanced through the space L, and further, the upper surface of the bracket part 11b' is lowered, and the space L is made larger than the length of the pivot shaft 10 to enable the pulling and insertion of the pivot shaft 10. Thus, the mud drainage in the boom bracket 11b is improved, and the exchange of the pivot shaft 10 accompanied by wear can be facilitated.