A fuel tank mounting mechanism which enhances backward visibility of an operator and performs the loading/unloading work of a fuel tank by a single motion to improve replacement workability. In the mechanism for mounting a fuel tank (30) on a counterweight (8) provided in the rear of an industrial vehicle, the front upper surface of the counterweight (8) is dented to form a recess (9) for receiving the fuel tank (30) in the width direction of the body. The recess (9) is provided with an arm (14) having a member (15) for fixing the fuel tank (30), and the arm (14) is supported rotatably for the body fixing portion through an arm rotating shaft (13) and arranged to rotate about the shaft (13) as the center of rotation to the right or left side in a vertical plane. The arm (14) hangs down from the body side when it rotates.