A discharge defect of a nozzle in an apparatus that forms an image on a medium by discharging liquid from a liquid discharge head based on image data is accurately detected. A discharge defect of a nozzle is detected by comparing data (reference data) of a reference image with read data obtained by reading an output image. In this case, a mark for matching the read pixel and the nozzle position with each other is recorded on the medium, and the correspondence relationship is specified by reading the mark. In addition, data items that are converted to have the nozzle resolution by performing a filtering process corresponding to the human visual characteristics for both the data of the reference image and the data of the read image are compared with each other, thereby detecting a discharge defect of a nozzle.