A washing method using a washing device for washing passages inside of a water treatment apparatus, which comprises a filter unit, a drain unit formed by water cocks (31, 32) admitting filtered water to provide purified water and drainpipes (33, 34), the washing method comprising: 1) a wash water inflow operation in which wash water comprising a detergent flows into circulation members (111-114, 117) of the washing device, the circulation members (111-114, 117) including - connection pipes connecting a circulation module of the washing device to the cocks (31, 32) and the drainpipes (33, 34) of the water treatment apparatus and - a circulation pipe connecting the circulation module to a purified water pipe at the outlet side of the filter unit or a purified water storage unit connected to the drain unit of the water treatment apparatus; 2) a circulation washing operation in which the circulation module is operated to circulate the wash water through the circulation members (111-114, 117) and through a plurality of passages of the water treatment apparatus so that the circulation members form an infinite loop together with the passages of the water treatment apparatus; 3) a wash water neutralization operation in which the wash water is neutralized by inputting a counteragent, according to the composition of the detergent, into the wash water after the circulation washing operation; 4) a draining operation in which neutralized wash water is drained off after the wash water neutralization operation is performed for the predetermined period of time; wherein in the draining operation, the neutralized wash water is drained off through a wash water drainpipe included in the circulation module by operating a passage switching unit, the passage switching unit being part of the circulation module and coupled to a portion where the wash water drainpipe and the circulation members (11-114, 117) are connected to thereby switch the passage of wash water, and a circulation pump included in the circulation module and connected to the circulation members to flow the wash water through the passages of the water treatment apparatus or through the wash water drainpipe; and 5) a cleaning operation in which the passages of the water treatment apparatus are cleaned by supplying purified water to the plurality of passages of the water treatment apparatus and draining off the purified water after the draining operation is performed, wherein the plurality of passages of the water treatment apparatus are divided into groups comprising at least one passage, and the wash water circulates through the groups in a sequential manner to thereby wash the plurality of passages. 1) a wash water inflow operation in which wash water comprising a detergent flows into circulation members (111-114, 117) of the washing device, the circulation members (111-114, 117) including - connection pipes connecting a circulation module of the washing device to the cocks (31, 32) and the drainpipes (33, 34) of the water treatment apparatus and - a circulation pipe connecting the circulation module to a purified water pipe at the outlet side of the filter unit or a purified water storage unit connected to the drain unit of the water treatment apparatus; - connection pipes connecting a circulation module of the washing device to the cocks (31, 32) and the drainpipes (33, 34) of the water treatment apparatus and - a circulation pipe connecting the circulation module to a purified water pipe at the outlet side of the filter unit or a purified water storage unit connected to the drain unit of the water treatment apparatus; 2) a circulation washing operation in which the circulation module is operated to circulate the wash water through the circulation members (111-114, 117) and through a plurality of passages of the water treatment apparatus so that the circulation members form an infinite loop together with the passages of the water treatment apparatus; 3) a wash water neutralization operation in which the wash water is neutralized by inputting a counteragent, according to the composition of the detergent, into the wash water after the circulation washing operation; 4) a draining operation in which neutralized wash water is drained off after the wash water neutralization operation is performed for the predetermined period of time; wherein in the draining operation, the neutralized wash water is drained off through a wash water drainpipe included in the circulation module by operating a passage switching unit, the passage switching unit being part of the circulation module and coupled to a portion where the wash water drainpipe and the circulation members (11-114, 117) are connected to thereby switch the passage of wash water, and a circulation pump included in the circulation module and connected to the circulation members to flow the wash water through the passages of the water treatment apparatus or through the wash water drainpipe; and 5) a cleaning operation in which the passages of the water treatment apparatus are cleaned by supplying purified water to the plurality of passages of the water treatment apparatus and draining off the purified water after the draining operation is performed,