PROBLEM TO BE SOLVED: To provide a control method for a process liquid for thinning a resin layer in a method of reducing a film thickness of a photosensitive resin layer by use of a process liquid for thinning a resin layer, in which an ability of reducing a film thickness of the process liquid for thinning a resin layer, which is an alkaline aqueous solution having a high concentration, can be more stably maintained at a given level.SOLUTION: The control method for a process liquid for thinning a resin layer is applied to a method of reducing a film thickness of a photosensitive resin layer, which includes steps of forming a photosensitive resin layer on a substrate surface and a step of reducing a film thickness of the photosensitive resin layer by use of the process liquid for thinning a resin layer, in this order. In the control method, the following methods are used together: a method of replenishing the process liquid with a given amount of a first replenisher in accordance with an amount of processed
