In order to reduce the size of an insertion part while avoiding the action of excessive stress in each part, a bending mechanism (5) according to the present invention is provided with: a long and narrow support member (6)； a swinging member (7) that is supported by the distal end of the support member so as to be capable of swinging about an axis line that intersects the longitudinal axis； drive force transmission members (8a, 8b) disposed along the longitudinal axis of the support member, the drive force transmission members transmitting drive force applied at a base end and causing the swinging member to swing； and a stress regulation part (15) that regulates the stress generated in the drive force transmission members, at each swing position of the swinging member relative to the support member, so that the stress does not exceed a prescribed threshold value. The drive force transmission members are provided with first transmission members (9a, 9b) by which the drive force transmission members connect to the swinging member, and second transmission members (14a, 14b) disposed farther toward the base-end side than are the first transmission members. The stress regulation part is provided with a movable member (16) that moves in a prescribed direction when the first and second transmission members move relatively in the longitudinal direction, and a biasing member (17) that biases in a direction to obstruct the motion of the movable member.L'invention concerne un mécanisme de flexion qui permet de réduire la taille d'une partie d'insertion tout en évitant l'action d'une contrainte excessive dans chaque partie. Le mécanisme de flexion (5), selon la présente invention, est pourvu : d'un élément support long et étroit (6) ； d'un élément oscillant (7) qui est porté par l'extrémité distale de l'élément support de façon à pouvoir osciller autour d'une ligne d'axe qui coupe l'axe longitudinal ； d'éléments de transmission de force d'entraînement (8a, 8b) disposés le long
