An extremely small diameter endoscope includes comprises: an endoscope body having a distal end nozzle； an imaging unit disposed in the endoscope body and adapted for capturing an image； an endoscope insertion part having a proximal end portion and a projection end portion, the proximal end portion connected to the imaging unit, the endoscope insertion part passing through the distal end nozzle, the projection end portion to be positioned in a root canal of a tooth or in a periodontal pocket； and endoscope insertion part insertion means adapted for inserting the endoscope insertion part into an observation portion in the root canal of the tooth or in the periodontal pocket with an end portion of the distal end nozzle kept abutted against a surrounding portion of the observation portion.
