The present invention provides an extremely small diameter endoscope which provides an improved operability in introducing an endoscope insertion part into a root canal or a periodontal pocket and has a function of preventing damages to the endoscope insertion part while it is not being introduced into the root canal or the periodontal pocket. An extremely small diameter endoscope of the present invention comprises: an endoscope body (2) having a distal end nozzle (11)； an imaging unit (4) disposed in the endoscope body (2) and adapted for capturing an image； an endoscope insertion part (12) having a proximal end portion and a projection end portion, the proximal end portion connected to the imaging unit (4), the endoscope insertion part (12) passing through the distal end nozzle (11), the projection end portion to be positioned in a root canal (22) of a tooth (21) or in a periodontal pocket (24)； and endoscope insertion part insertion means (6) adapted for inserting the endoscope insertion part (12) into an observation portion in the root canal (22) of the tooth (21) or in the periodontal pocket (24) with an end portion of the distal end nozzle (11) kept abutted against a surrounding portion of the observation portion.
