1. A medical image processing device comprising: a processor comprising hardware, the processor being configured to implement: an input section to which a biological mucous membrane image obtained by picking up an image of a biological mucous membrane is inputted; a region extracting section configured to extract a mucous membrane microstructure region corresponding to a mucous membrane microstructure, which is a microscopic structure included in the biological mucous membrane, from the biological mucous membrane image inputted to the input section; an imaginary mucous membrane microstructure setting section configured to set an imaginary mucous membrane microstructure region on the biological mucous membrane image; a closed region identifying section configured to identify at least one closed region regarded as being surrounded by the mucous membrane microstructure region extracted by the region extracting section and the imaginary mucous membrane microstructure region set by the imaginary mucous membrane microstructure setting section; and a unit region setting section configured to set a unit region, which is a region set as one unit in terms of biological histology, on the basis of the mucous membrane microstructure region extracted by the region extracting section and the closed region identified by the closed region identifying section.