[PROBLEMS] To provide: a method of cleaning an endoscope by which the endoscope can be effectively cleaned while being inhibited from deteriorating; and a cleaning apparatus and endoscope-cleaning apparatus. [MEANS FOR SOLVING PROBLEMS] An electrolytic cell (11) can produce from tap water an acidic water having a pH of 2-5 and a residual chlorine concentration of 50-300 ppm. An acidic-water tank (14) can store the acidic water, part of which can be supplied to an acidic-water dilution tank (17) and other part of which can be supplied to a high-concentration-water supply part (20). A diluting-water supply part (16) can supply diluting water to the acidic-water dilution tank (17). In the acidic-water dilution tank (17), diluting water from the diluting-water supply part (16) is added to that part of the acidic water which has been sent from the acidic-water tank (14), whereby a diluted acidic water having a pH of 2.3-5.7 and a residual chlorine concentration of 10-60 ppm can be prepared. The high-concentration-water supply part (20) can feed that other part of the acidic water which has been supplied from the acidic-water tank (14) to the opening of an endoscope channel through a channel connection opening (20c).