A fuel tank device capable of detecting consistency includes a fuel storage tank and a consistency detecting box. The fuel storage tank provides fuel needed by a fuel cell as a fuel supply source for the fuel cell. A consistency detecting passage is disposed near the bottom of the fuel storage tank for the fuel flowing outward to the consistency detecting box. The consistency detecting box further includes a signal emitting component, which is disposed in the consistency detecting box and capable of emitting a signal piercing the fuel in the consistency detecting box, a signal receiving component, which is disposed in the consistency detecting box and capable of collecting the signal after piercing the fuel in the consistency detecting box and a consistency detecting component, which is attached to the consistency detecting box, compares the emitted signal with the collected signal for detecting a consistency of the fuel.