This invention is a three-dimensional image drawing apparatus that comprises a frame buffer for storing display data corresponding to a display screen in a display apparatus, and a two-dimensional drawing circuit for performing coordinate operations in order to draw specific two-dimensional images in the frame buffer. The apparatus also uses a Z buffer for storing depth data in order to perform three-dimensional drawing to the frame buffer, and a three-dimensional straight line drawing circuit for making three-dimensional drawings to the frame buffer based on the depth data stored in the Z buffer. The apparatus also uses a memory control circuit for controlling access to the buffers, and a bit number altering circuit for altering the number of bits per pixel in the coordinate operations of the two-dimensional drawing circuit either to a number of bits corresponding to the drawing data in the frame buffer or to a number of bits corresponding to the depth data in the Z buffer. The areas of both the frame buffer and Z buffer here are established in the same physical memory.