Provided is a method for manufacturing a microbicide having high microbicidal performance for eradicating microbes. This method for manufacturing a microbicide comprises: a step for preparing an inorganic aqueous solution containing an inorganic component having seawater as a raw material thereof, an ozone mixing step for mixing ozone into the inorganic aqueous solution, and a stirring step for stirring the inorganic aqueous solution mixed with ozone and passing through a bubble generation nozzle wherein, the temperature of the inorganic aqueous solution in the ozone mixing step and the stirring step is 0° C. to 30° C., and when the amount of inorganic aqueous solution treated in the ozone mixing step and the stirring step is defined as X liters and the treatment rate of the ozone mixing step and the stirring step is defined as Y liters/minute, then the microbicide is manufactured by alternately repeating the ozone mixing step and the stirring step for A·X/Y minutes (where A is 30 or more).
