There is provided a gas tank that suppresses an increase in pressure of a gas filled therein. A hydrogen tank comprises a tank main body filled with hydrogen, a thermofoamable heat-insulating layer provided on the external surface of the tank main body, a relief valve which is provided at the left end of the tank main body and which reduces the pressure in the tank main body by releasing the hydrogen in the tank main body, and a liner which extends from the relief valve toward the right end and which detects an increase in temperature. The relief valve is thermally connected to the liner, and is actuated based on heat transferred from the liner in order to release the hydrogen in the tank main body, thereby reducing the pressure in the tank main body.