A fuel tank system stores liquefied gas fuel to be supplied to an engine. The liquefied gas fuel is refueled to the fuel tank system by a fueling equipment. The fuel tank system includes: a first tank part having a connector portion connected with the fueling equipment and defining a first storage chamber into which the liquefied gas fuel sent from the fueling equipment flows; a second tank part defining a second storage chamber that stores the liquefied gas fuel separately from the first storage chamber; and a sending part that raises a pressure of the liquefied gas fuel stored in the first tank part and that sends the liquefied gas fuel with the raised pressure to the second tank part.