A hydraulic oil tank (17) is supported by a tank support base (18) and a tank support base (19) at a position higher than that of an upper surface (8A) of a right side frame (8) . Also, a space portion (23) with an opening (23A) corresponding to an outer side in the right-and-left direction of an upper revolving structure (3) is provided between a hydraulic oil tank attachment site (9B) and a hydraulic oil tank attachment site (10B), and a bottom surface portion (17E) of the hydraulic oil tank (17). In addition, the opening (23A) of the space portion (23) is provided with an opening/closing cover (25D) openably closing the opening (23A).