To continuously and stably manufacture a diluted solution adjusted in a prescribed concentration.SOLUTION: A diluted solution manufacturing method includes: a step of supplying a first liquid to a first pipe 11; a step of adding a second liquid in a tank 12a to the first liquid flowing in the first pipe 11 via a second pipe 13; a step of measuring the first liquid flowing in the first pipe 11 or a flow amount of a diluted solution; a step of measuring a concentration of the diluted solution; and a step of adjusting an adding amount of the second liquid to the first liquid so that the concentration of the diluted solution becomes a prescribed concentration. The step of adjusting the adding amount of the second liquid is switched to a first adjustment mode for adjusting the adding amount of the second liquid on the basis of the flow amount and the concentration measured and a second adjustment mode for adjusting the adding amount of the second liquid on the basis of the flow amount and the concentration measured, and executes the adjustment of the adding amount of the second liquid. The switching from the first adjustment mode to the second adjustment mode is performed when the flow amount measured satisfies a prescribed condition.SELECTED DRAWING: Figure 1
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