PURPOSE: To remove water in an oil tank perfectly and simply, by arranging a container accomodating an element using a water absorbing polymer in the oil tank in a detachable manner.

CONSTITUTION: The oil in an oil tank 6 is recirculated by driving an oil recirculating pump and, as the oil recirculated and stirred, water and water droplets are impinged on an element comprising a water absorbing polymer in a cylindrical container 2 to be directly absorbed thereby. Hereinafter, this operation is succeeded to obtain the oil from which water is perfectly removed. In addition, the replacement of the element can be carried out in such a manner that a lid 7 is detached from a ring body 5 along with the container 2, a lid 3 and a chain 4 and, after the chain 4 is detached from the lid 3, the lid 3 is detached from the container 2. After the element 1 is replaced, operations reverse to the each above described operations are carried out to return the element to the original condition.