To provide a tank support device possible to easily and continuously secure rigidity.

This support device for supporting a cylindrical tank 2 on a floor plate 8 of a vessel laterally and longitudinally provided with reinforcing members 9 such as girders and longitudinals in a lower surface is formed of a belt-like plate part 3 to be wound around the peripheral surface of the cylindrical tank 2, a ring-like reinforcing plate part 4 fitted to a lower edge of the belt-like plate 3, projecting in the horizontal direction, a conical plate part 5 formed into an inverted conical shape and connected to a lower surface of the reinforcing plate part 4 and a support plate part 6 formed into a nearly triangle shape with a view from side and provided in the peripheral surface of the conical plate part 5 on all sides, projecting in the vertical direction. When installing, the support plate part 6 is arranged in the direction of the reinforcing member 9 provided in the lower surface of the floor plates 8 of the vessel.