Cleaning system for piping

Method for cleaning piping (P) including the steps of supplying an acid (A) to a cleaning water (S) to prepare a cleaning water (S) having pH of 4 or lower, mixing ozone gas to the cleaning water (S), and passing the cleaning water (S) through the piping (P) to be cleaned, and a cleaning system (1) for piping (P), the system (1) including a reservoir (10) which retains cleaning water (S), an acid providing means (30) supplies an acid (A) to the cleaning water (S), an ozone generation means (20) which generates ozone gas, a circulation flow path (120) including a circulation pump (420) which connects the reservoir (10) and the ozone generation means (20) in the form of a closed circuit, and circulates the cleaning water (S) between the reservoir (10) and ozone generation means (20), a conduction flow path (140) including a conveying pump (440) which communicates the reservoir (10) and the piping (P) to be cleaned, and conveys the cleaning water (S) retained in the reservoir (10) through the piping (P) to be cleaned.