A washing device comprising: a washing space arranged to receive items for washing; and a descaling solution supply device arranged to supply a descaling solution, the descaling solution supply device comprising: a main flow path arranged to guide water to the washing space; a solution accommodating unit arranged to accommodate a descaling agent, wherein the solution accommodating unit comprises a solution outlet (lllh) arranged to discharge water, in which the descaling agent is dissolved, to the main flow path; a solution discharge valve arranged to open and close the solution outlet; and a branch flow path which branches from the main flow path and is arranged to supply water to the solution accommodating unit, wherein the branch flow path comprises an atmosphere communicating port arranged to communicate with the atmosphere, wherein the branch flow path includes: an upstream branch flow path arranged to upwardly guide water flowing in from the main flow path, and a downstream branch flow path arranged to downwardly guide water received from the upstream branch flow path, and wherein the position of the atmosphere communicating port is such that the water level in the downstream branch flow path 123b is restricted to a predetermined height by atmospheric pressure applied through the atmosphere communicating port when supply of water through the branch flow path is shut off after the solution accommodating unit is filled with water and the solution outlet (lllh) is closed by the solution discharge valve, characterized in that the area of an outlet of the upstream branch flow path is less than the area of an inlet of the downstream branch flow path. a washing space arranged to receive items for washing; and a descaling solution supply device arranged to supply a descaling solution, the descaling solution supply device comprising: a main flow path arranged to guide water to the washing space; a solution accommodating unit arranged to accommodate a descaling agent, wherein the solution accommodating unit comprises a solution outlet (lllh) arranged to discharge water, in which the descaling agent is dissolved, to the main flow path; a solution discharge valve arranged to open and close the solution outlet; and a branch flow path which branches from the main flow path and is arranged to supply water to the solution accommodating unit, wherein the branch flow path comprises an atmosphere communicating port arranged to communicate with the atmosphere, wherein the branch flow path includes: an upstream branch flow path arranged to upwardly guide water flowing in from the main flow path, and a downstream branch flow path arranged to downwardly guide water received from the upstream branch flow path, and wherein the position of the atmosphere communicating port is such that the water level in the downstream branch flow path 123b is restricted to a predetermined height by atmospheric pressure applied through the atmosphere communicating port when supply of water through the branch flow path is shut off after the solution accommodating unit is filled with water and the solution outlet (lllh) is closed by the solution discharge valve, characterized in that the area of an outlet of the upstream branch flow path is less than the area of an inlet of the downstream branch flow path. a main flow path arranged to guide water to the washing space; a solution accommodating unit arranged to accommodate a descaling agent, wherein the solution accommodating unit comprises a solution outlet (lllh) arranged to discharge water, in which the descaling agent is dissolved, to the main flow path; a solution discharge valve arranged to open and close the solution outlet; and a branch flow path which branches from the main flow path and is arranged to supply water to the solution accommodating unit, wherein the branch flow path comprises an atmosphere communicating port arranged to communicate with the atmosphere, wherein the branch flow path includes: an upstream branch flow path arranged to upwardly guide water flowing in from the main flow path, and a downstream branch flow path arranged to downwardly guide water received from the upstream branch flow path, and wherein the position of the atmosphere communicating port is such that the water level in the downstream branch flow path 123b is restricted to a predetermined height by atmospheric pressure applied through the atmosphere communicating port when supply of water through the branch flow path is shut off after the solution accommodating unit is filled with water and the solution outlet (lllh) is closed by the solution discharge valve, characterized in that the area of an outlet of the upstream branch flow path is less than the area of an inlet of the downstream branch flow path. an upstream branch flow path arranged to upwardly guide water flowing in from the main flow path, and a downstream branch flow path arranged to downwardly guide water received from the upstream branch flow path, and wherein the position of the atmosphere communicating port is such that the water level in the downstream branch flow path 123b is restricted to a predetermined height by atmospheric pressure applied through the atmosphere communicating port when supply of water through the branch flow path is shut off after the solution accommodating unit is filled with water and the solution outlet (lllh) is closed by the solution discharge valve, characterized in that the area of an outlet of the upstream branch flow path is less than the area of an inlet of the downstream branch flow path.