PROBLEM TO BE SOLVED: To improve the holding force of an arm cylinder of a bracket for mounting arm cylinder by welding the bracket for mounting arm cylinder close to a base part at the rear end of a front boom. SOLUTION: Bosses 28, 29 for insertion of upper and lower connection pins, and a partition wall board 33 having a curved shape, as seen from the side thereof, extending between a right and left side boards 27L, 27R are welded to the inner side of a base part 24 at the rear end of a front boom 23. And reinforcing lapping plates 35, 36 are welded to the boom 23 so as to extend from the lower end of the part 24 to the rear end surface and from the upper surface of the part 24 to the rear end surface, respectively. Further a bracket 31 for mounting arm cylinder is welded close to the boss 29 for insertion of connection pin on the lower side of the base part 24.