A parenchymatous tissue image and a blood vessel image are synthesized as follows. First, a tomographic image of a specific cross section of the parenchymatous tissue image is synthesized with the blood vessel image closer to the viewpoint along the Z-axis than the tomographic image. Then, the resulting image is displayed. Thereafter, the operator inputs specific data from operation unit including a mouse and a keyboard, thereby changing the position of a displayed tomographic image along the Z-axis. This enables the operator to sequentially display two-dimensional tomographic images (B-mode images) constituting a three-dimensional parenchymatous tissue image as if to turn over the pages of a book.