A construction machine, comprising: a lower traveling structure, an upper revolving structure rotatably mounted on said lower traveling structure, a working mechanism provided on the front side of said upper revolving structure capable of moving upward/downward, and a counterweight provided on the rear side of said upper revolving structure in order to keep balance with said working mechanism, wherein said upper revolving structure is provided with: a revolving frame forming a support structural body and on which a supporting bracket is provided on a front end; an engine located on the front side of said counterweight and mounted on said revolving frame; a swing post provided on said supporting bracket of said revolving frame and supporting said working mechanism swingably in the left-right direction; a swing cylinder provided between said swing post and said revolving frame and swinging said swing post in the left-right direction; an operating oil tank located in the vicinity of said swing cylinder and on the front side of said engine and provided on said revolving frame and storing an operating oil to be supplied to a hydraulic actuator; and a fuel tank located on the front side of said operating oil tank and provided on said revolving frame and storing a fuel to be supplied to said engine, characterized in that: a mounting member is provided on said revolving frame across a part of said swing cylinder and located on the lower side of said fuel tank; a vertical restraining element (20, 20') for restraining said fuel tank in the front-rear direction and the vertical direction is detachably provided between an upper-part position of said operating oil tank and said mounting member; and a lateral restraining element for restraining said fuel tank in the left-right direction is detachably provided between a side surface portion on the outside in the left-right direction of said operating oil tank and said mounting member. a lower traveling structure, an upper revolving structure rotatably mounted on said lower traveling structure, a working mechanism provided on the front side of said upper revolving structure capable of moving upward/downward, and a counterweight provided on the rear side of said upper revolving structure in order to keep balance with said working mechanism, wherein said upper revolving structure is provided with: a revolving frame forming a support structural body and on which a supporting bracket is provided on a front end; an engine located on the front side of said counterweight and mounted on said revolving frame; a swing post provided on said supporting bracket of said revolving frame and supporting said working mechanism swingably in the left-right direction; a swing cylinder provided between said swing post and said revolving frame and swinging said swing post in the left-right direction; an operating oil tank located in the vicinity of said swing cylinder and on the front side of said engine and provided on said revolving frame and storing an operating oil to be supplied to a hydraulic actuator; and a fuel tank located on the front side of said operating oil tank and provided on said revolving frame and storing a fuel to be supplied to said engine, characterized in that: a revolving frame forming a support structural body and on which a supporting bracket is provided on a front end; an engine located on the front side of said counterweight and mounted on said revolving frame; a swing post provided on said supporting bracket of said revolving frame and supporting said working mechanism swingably in the left-right direction; a swing cylinder provided between said swing post and said revolving frame and swinging said swing post in the left-right direction; an operating oil tank located in the vicinity of said swing cylinder and on the front side of said engine and provided on said revolving frame and storing an operating oil to be supplied to a hydraulic actuator; and a fuel tank located on the front side of said operating oil tank and provided on said revolving frame and storing a fuel to be supplied to said engine, characterized in that: a mounting member is provided on said revolving frame across a part of said swing cylinder and located on the lower side of said fuel tank; a vertical restraining element (20, 20') for restraining said fuel tank in the front-rear direction and the vertical direction is detachably provided between an upper-part position of said operating oil tank and said mounting member; and a lateral restraining element for restraining said fuel tank in the left-right direction is detachably provided between a side surface portion on the outside in the left-right direction of said operating oil tank and said mounting member.