To provide a washing device which is capable of reusing substitution gas for substituting the gas in a washing tank.

A washing system 1 washes objects 500 to be washed by supercritical carbon dioxide or liquid state carbon dioxide. The washing system 1 is provided with a washing tank 310 for accommodating the objects 500, a supply tube 330 for supplying gas including the substitution gas to be substituted with the gas in the washing tank 310 into the washing tank 310, a discharge tube 340 for discharging the mixed gas including the substitution gas and the gas inside the washing tank 310 from the washing tank 310, and a returning channel 370 which is connected with the discharge tube 340 and the supply tube 330 for returning the mixed gas discharged from the discharge tube 340 to the washing tank 310 via the supply tube 330.