PURPOSE: To efficiently cleanse the green solution pipings of a soda-recovering boiler installation.

CONSTITUTION: A communicating pipe (13) for communicating plural system green solution-transferring pipes (2a), (2b) with each other through changeover valves (12a), (12b) is disposed at a place near to a dissolving tank (1), and weak solution-jetting pipes (17a), (17b) branched from the communicating pipe (13) and used for jetting a weak solution into the inlets of green solution-circulating pipes (7a), (7b) in the dissolving tank (1) is also disposed. Thus, when the weak solution is flowed back into a part of the plural system green solution-transferring pipes (2a), (2b), e.g. the pipe (2b), to cleanse the pipe, the whole amount of the weak solution can be jetted into the inlet of the green solution-circulating pipe (7b) in the dissolving tank (1) through the communicating pipe (13) and the weak solution-jetting piping (17b). The part whose cleansing has been difficult can thereby effectively be cleansed. Since most of the weak solution is jetted into the green solution-circulating pipe (7b), the cleansing effect is remarkably improved in comparison with conventional cases in which the cleansing treatments have been performed with small amounts of the weak solution.