A scanning position of laser light applied to a projection plane is detected, and based on the detected scanning position, a synchronization signal is generated to designate a scan timing. A correction region detection portion detects whether or not a part corresponding to a prescribe region of an image on the projection plane is scanned by laser light, based on the generated synchronization signal. When the part corresponding to the prescribed region is scanned, a correction amount derivation portion corrects a gradation signal based on the difference between the detected quantity of laser light and a prescribed quantity of light to be output from the laser for displaying the image in the prescribed region.