To provide a washing device for effectively washing an object to be washed with an additive and preventing the additive from remaining.

A washing system 1 is provided with a pressurizing means, a pressure reduction means, a washing tub 10, a carbon dioxide supply means, a carbon dioxide discharge means, a pressure adjustment means, and a control part 91. The pressurizing means pressurizes washing liquid so that the additive 200 becomes at least soluble. The pressure reduction means reduces the pressure of the pressurized washing liquid and foam the additive 200. The control part 91 executes the process of controlling the carbon dioxide supply means so as to supply carbon dioxide in a supercritical or liquid state to the cleaning tub 10 after a fiber structure 100 is washed by the washing liquid containing the foamed additive 200 and the process of controlling the carbon dioxide discharge means so as to discharge the supplied carbon dioxide, and also controls the pressure adjustment means so as to adjust pressure inside the washing tub 10 to a prescribed pressure in order to make the additive 200 at least soluble in the carbon diode in the supercritical or liquid state discharged from the washing tub 10.