An endoscope bending operation mechanism includes: an operation lever with one end being rotatably held; a frame configured to rotate with the operation lever; a fixing member rotatably supporting the frame; a rotation shaft coupled with an end portion of one of the frame and the fixing member; a bearing portion provided on the other of the frame and the fixing member and having a hole rotatably holding the rotation shaft; and a position defining portion configured to abut a part of the other of the frame and the fixing member to define a position of the rotation shaft. The position of the rotation shaft is adjustable, and by adjusting the position of the rotation shaft, a clearance between the bearing portion and the position defining portion in a direction along a central axis is adjusted.