1. A medical image generating device for generating an observation image, which is a medical image generating device for generating a predetermined observation image on the basis of three-dimensional image data in which predetermined signal values obtained with a diagnostic imaging apparatus are associated respectively with a plurality of image-constituting elements that make up a three-dimensional image model of a body to be observed, the device comprising: selection means for selecting an image-constituting element group corresponding to a predetermined biological tissue, from among the plurality of image-constituting elements; signal value adjusting means for replacing each signal value associated with the selected image-constituting element group with a converted signal value obtained by a predetermined conversion processing; and image constructing means for constructing the observation image by maximum intensity projection or minimum intensity projection on the basis of the three-dimensional image data after replacement of the signal values.