To easily apply a method of easily washing an air preheater with water independently from washing of an electric dust collector to existing equipment in a boiler exhaust gas system.

In this washing method of the air preheater in the boiler exhaust gas system including: the air preheater 1 receiving an exhaust gas discharged from a combustion furnace and exchanging heat with the combustion air supplied to the combustion furnace; and the electric dust collector 3 receiving the exhaust gas discharged from the air preheater 1 through flues 4, 13, 21-25 and removing soot dust of the exhaust gas, blocking means 41, 5 are disposed in the flues 4, 13, 21-25, so that the air preheater 1 is washed after blocking the flues 4, 13, 21-25 by the blocking means 41, 5.