The present invention relates to a video signal line drive circuit of a display device.An object of the present invention is to, when gradation conversion is performed in the display device using an error diffusion method, provide a display with a smooth gradation change even at boundaries of display blocks.In an error diffusion operation circuit (36) of a source driver unit (302), an error diffusion process is performed for each pixel based on, in addition to image data (Da) for a display block of the source driver unit (302), image data (Da) for areas near boundaries between the display block of the source driver unit (302) and display blocks of source driver units (301, 303) in previous and subsequent stages. A video signal is generated based on error-diffused image data (Db) generated by the error diffusion process.