To provide a construction method for a water purifying apparatus, which enables purified water to be as rapidly supplied as possible to a use point by shortening the construction period and start-up period of the water purifying apparatus.

In this construction method for the water purifying apparatus, the water purifying apparatus is assembled and constructed in the place of demand for the purified water. The construction method includes a process wherein a second water purifying apparatus (4), which can be moved before the construction of a first water purifying apparatus (2), is temporarily installed in the demand place for constructing the apparatus (2), a process wherein the purified water, which is manufactured by the apparatus (4), is applied to the use point (6), and a process wherein switching from the apparatus (4) to the apparatus (2) in the supply of the purified water to the use point (6) is accomplished after the completion of the construction of the apparatus (2). The purified water can be rapidly supplied by shortening the construction period and start-up period of the water purifying apparatus.