At least two (groups of) sensors forming an insertion detection sensor are offset from each other in the conveyance direction of a photosensitive material. An insertion speed, which is different for each manual insertion event, is obtained based on the time difference when sensors that have been offset from each other detect the leading end of the photosensitive material. The length of the photosensitive material in the conveyance direction thereof is accurately computed based on the insertion speed and other information. By correcting the errors due to the changeable insertion state of the photosensitive material caused by manual insertion by the operator, the process area of the photosensitive material, required for calculating the amount of replenisher to be replenished, is accurately obtained. Therefore, the amount of the replenisher is appropriately determined and the process capacity of the developer or the fixing solution can constantly be maintained at the satisfactory level.