PURPOSE: To improve the ignitability and flammability of a fuel oil by sinking a specified hydrophilic bag in a light fuel oil and making absorbed and removing moisture present in the oil.

CONSTITUTION: A hydrophilic bag contg. a water-adsorbent powder (a), a powder (b) which is a water-insoluble or hardly soluble basic compd. or reacts with water to produce a water-insoluble or hardly soluble basic compd., an acid-base indicator (c) which colors when the compd. (b) is brought into contact with water, and a weight (d) is sunk into a light fuel oil to make absorbed and remove moisture present in the oil. To make water absorbed and retained with a good balance, the powders are mixed by taking characteristics of each powder into consideration. In addition, to improve the water-absorbency, a small amt. of a surface active agent may be adsorbed on the surface of the water-absorbent powder.