A developing apparatus comprises: a photodetection unit, which emits detecting light toward the development area of the substrate to be developed within a scheduled time after the substrate to be developed is immersed into the developer solution; and a processing unit electrically coupled with the photodetection unit for determining the time interval which it takes for development to occur in the development area by means of the detecting light, and for determining that the developer solution is failed if the development time interval is determined to be out of the preset time range. A method for monitoring the developer solution is also provided.