A multibeam scanning optical apparatus incluses a controller which specifies laser diodes which are emitting light normally and arranged in a continuous row, a memory having a plurality of area in which image data are stored, a write-in controller which causes image data for each line to be written into only areas of memory corresponding to the specified laser diodes when any of the laser diodes suffers a failure, a read-out controller which causes the image data for each line to be read out simultaneously from the areas of memory corresponding to the specified laser diodes and a driver which performs light emission control only with respect to the specified laser diodes.