Provided are a method of cleaning an endoscope, a washing machine and a washing machine for endoscope, which can effectively clean an endoscope while preventing the endoscope from deterioration. The electrolytic cell 11 can produce from tap water acidic water with pH of 2 to 5 and the residual chlorine concentration of 50 to 300 ppm. A tank for acidic water 14 can store acidic water, in which part thereof can be fed to a tank of diluting acidic water 17 and the rest thereof can be fed to a feed section of concentrated water 20. A water feed section for dilution 16 can feed water for dilution to the tank of diluting acidic water 17. The tank of diluting acidic water 17 can add water for dilution from the water feed section for dilution 16 to part of the acidic water from the tank for acidic water 14 to adjust diluted acidic water to pH of 2.3 to 5.7 and the residual chlorine concentration of 10 to 60 ppm. The feed section of concentrated water 20 can feed the rest of acidic water fed from the tank for acidic water 14 through a channel connection port 20c to a channel port of an endoscope.