A penetrating hole for observation, which is provided in a tip body 20 that forms a tip surface 11a at the tip 11 of the insertion section 6, and whose opening shape is defined by a pair of long sides s1 facing each other and a pair of short sides s2 facing each other. The hole 22, a groove 50 for processing relief formed at four corners where the long side s1 and the short side s2 intersect each other when the observation through hole 22 is processed, and a plan view shape similar to the opening shape of the observation through hole 22. And has an observing lens 24 inserted into the observing through-hole 22 and an adhesive portion 51 provided between the inner peripheral surface of the observing through-hole 22 and the outer peripheral surface of the observing lens 24. In the endoscope 2, each groove 50 is formed so as to be in contact with the short side s2 and project from the long side s1 to the outside of the observation through hole 22.挿入部６の先端部１１に先端面１１ａを形成する先端部本体２０に設けられ、互いに対向する一対の長辺ｓ１と互いに対向する一対の短辺ｓ２とによって開口形状が規定された観察用貫通孔２２と、観察用貫通孔２２の加工時に長辺ｓ１と短辺ｓ２とがそれぞれ交わる四隅に形成された加工逃げ用の溝部５０と、観察用貫通孔２２の開口形状と相似する平面視形状をなし、観察用貫通孔２２に挿入された観察用レンズ２４と、観察用貫通孔２２の内周面と観察用レンズ２４の外周面との間に周設された接着部５１と、を有する内視鏡２において、各溝部５０を、短辺ｓ２に接し、且つ、長辺ｓ１から観察用貫通孔２２の外方に突出するように形成する。
