A fuel tank and a cap device for the fuel tank. The fuel tank is connected to a fuel cell and includes a tank body having a fuel opening and a pin hole for maintaining an inside pressure, a floater positioned on a surface of the fuel stored in the tank body, and an air pipe connected to the pin hole and an upper part of the floater. The cap device includes a cap body having an inside transfer path for connecting an inlet opening for introducing the fuel and an outside transfer pipe, a fixed outer cap including a connector for fixing a discharge opening into which the transfer pipe is inserted, and an elastic opening/closing section in which the cap body is elastically compressible by the connector, and wherein the inside transfer path opens during the compression.