A DDA 34 notifies the coordinates of a rasterized pixel to an exclusive control part 40, acquires a unique identification number associated with the pixel position from the exclusive control part 40, and adds the identification number to pixel data and supplies it to a shader 20. A plurality of shader pipes 22 in the shader 20 perform arithmetic processing for rendering pixels in parallel, and writes the processing results to a frame buffer 50. When performing an arithmetic instruction that requires exclusive control over a pixel, each shader pipe 22 issues a request to lock the pixel by notifying the identification information added to that pixel to the exclusive control part 40. If the lock request is accepted, the shader pipe 22 performs the arithmetic processing on that pixel. If the lock request is rejected, the shader pipe suspends and puts the arithmetic processing on that pixel into a wait state, and executes arithmetic processing on another pixel in the interim.