A tank for storing gas is provided with a dye capsule layer including capsules containing dye or with an impact recording layer formed on the outer surface of the tank body. The impact recording layer has, for example, a hear-insulating material layer formed on the outer surface of the tank body, having a heat-insulating function, and capable of holding deformation, a dye capsule layer formed on the outer surface of the heat-insulating layer and including capsules containing dye, and a dye absorbing layer formed on the outer surface of the dye capsule layer and capable of absorbing dye. By an impact, the dye released from the dye capsule layer is absorbed into a dye absorbing layer of the tank. The dye exudes on the surface of the dye absorbing layer and spreads. The degree of impact given to the tank is estimated from the degree of spread of the dye or the degree of density of the dye.