Abstract: Hereby established is a method of disinfection by ozone which is safe, clean, more reliable, and more efficient than prior methods. The method comprises a step of calculating the amount of ozone which would be required to solely disinfect bacteria if there were no co-existing materials, a step of calculating the amount of ozone which will be consumed by actual co-existing materials and a step of applying the total value of the two ozone amounts in proper forms, such as, gas or liquid, etc., preferably in the form of aqueous solution, to the object to be disinfected. This method can be consistently adopted for disinfection on very wide varieties of bacteria.