To prevent the deck of a single deck type floating cover from being deformed due to sloshing (liquid surface splash) which occurs in a floating type storage tank due to vibration as of an earthquake, thereby reducing collapse breakage of the floating cover which is caused by excessive circumferential compression stress of a pontoon, and breakage of the floating cover caused by excessive partial stress in a joint between the deck and the pontoon.

The floating cover type storage tank includes a tank body open in the top, and the floating cover floating on the surface of a stored liquid and freely slid upward and downward such that its periphery is in contact with the internal faces of side plates. In the floating type storage tank, sufficiently rigid, radial fortifying members 16 or a sufficiently rigid circumferential fortifying member 17 is disposed in the deck 9 of the single deck type floating cover. In particular, the radial fortifying members 16 have such a structure that the radial bending moment of each of the radial fortifying members 16 can be smoothly transmitted to the pontoon 10 in the joint between each radial fortifying member 16 and the pontoon 10.