A two-beam scanning optical apparatus includes a first beam generating device, a second beam generating device, a judging unit which judges whether the second beam generating device is operating normally or not, a raster signal processing unit which outputs a raster signal each time if the judging unit judges that the second beam generating device is operable, and which outputs a raster signal every other time if the judging unit judges that the second beam generating device is not operable and a control unit which performs image formation by using the first beam generating device and the second beam generating device based on the raster signal output every other time, and which performs image formation by using only the first beam generating device based on the raster signal output each time.