PROBLEM TO BE SOLVED: To provide a fuel tank system, which is capable of correctly displaying a total fuel quantity in a fuel tank on a display means, even right after the completion of refueling.SOLUTION: The fuel tank system 10 includes: a fuel tank 12 having a main tank 20 and a sub-tank 22 for storing fuel; a fuel supply pipe 32 for supplying the fuel from outside into the main tank 20; a siphon tube 24 for transporting the fuel in the main tank 20 to the sub-tank 22 so that a liquid level ha of the fuel in the main tank 20 may be equalized with a liquid level hb in the sub-tank 22; a sender unit 52 for detecting a liquid level ha of the fuel in the main tank 20; an estimation part 66 for estimating the total fuel quantity in the fuel tank 12, based on a detection value of the sender unit 52 before completing a fuel transportation from the main tank 20 to the sub-tank 22; and a fuel meter 14 for displaying a total fuel quantity Qt estimated in the estimation part 66.