The invention provides a fuel gas tank installation structure of a vehicle body which can fully ensure fuel gas tank configuration space, and prevent reduction of the rigidity of regions around the fuel gas tank configuration space of the vehicle body, can reduce the height of a cargo platform, and can lower the center of gravity of a vehicle. The invention relates to the fuel gas tank installation structure including a fuel gas tank (5) having a cylindrical outer peripheral surface is arranged between two longitudinal beams (2) in the length direction; the fuel gas tank installation structure includes an assistant cross beam (6) which is positioned above the fuel gas tank (5) and connects the two longitudinal beams (2); a lower supporting member (12) having a tank supporting portion (13) for supporting the fuel gas tank (5) from the lower part. The tank supporting portion (13) bends to form a tank contact surface (13a) corresponding to the outer periphery of the fuel gas tank (5), the lower supporting member (12) is arranged on the assistant cross beam (6) in a state that the tank contact surface (13a) contacts with the outer peripheral surface of the fuel gas tank (5), and then the fuel gas tank (5) is arranged on the vehicle body (1).