To solve problems where a manhole buried in a road or around a building is moved upward and protruded above a surface of a ground (a street or a roadway) due to fluidization of the ground generated by a large earthquake and thereby causing a major hazard to moving traffic or the like and, at the same time, inflicting enormous damage as a result of disturbance and loss of original functions such as drainage and the like when several kinds of pipes connected in the manhole under the ground for water supply, sewerage, electricity supply and communication are broken or moved upward by huge pressure.

When a liquefaction phenomenon of the ground is generated by an earthquake, a device for preventing flotation of a manhole 1 due to the liquefaction phenomenon prevents the manhole 1 from the floatation by reducing pressure to float the manhole 1 in a manner that: introduces water content of liquefied or fluidized soil around a bottom plate 9 and surroundings of a box body 3 of a manhole 1 into both a main drainage pipe 8 and a sub-drainage pipe 11 through many small holes 7 provided therewith; and drains the water content into the box body 3 through the main drainage pipe 8, a drain pipe 15 and the sub-drainage pipe 11.