PURPOSE: To provide a removing device for dissolving oxygen to decrease the amt. of the oxygen without increasing the height of layers of packing material or without increasing the supply amt. of inert gas to be used.

CONSTITUTION: This device to remove oxygen dissolving in raw water is equipped with a reservoir 6 for the treated water in the lower part of a deaeration tower 1 having a gas-liquid contact part 2. A distributor 7 for inert gas is disposed at the position in the treated water in the reservoir 6. Inert gas is distributed from the distributor 7 in the treated water in the reservoir 6.