There are provided a lesion part volume measuring method and an endoscopic diagnostic apparatus capable of easily detecting the volume of a lesion part without using a special treatment instrument. This problem is solved by detecting the position of a distal end portion of a scope hood from an image captured by an endoscope to which the scope hood is attached, finding the volume of the internal space of the scope hood, which is formed by the scope hood and the distal end surface of an insertion part, from the position of the distal end portion of the scope hood and the model of the endoscope and/or the model of the scope hood, and injecting water into the scope hood and detecting the volume of a lesion part from the difference between the water injection amount and the volume of the internal space of the scope hood.
