An operation mechanism for an insertion device includes a tabular frame body 20 fixed in an operation section, a proximal end of an insertion section being fixed to the frame body 20, the frame body 20 turnably supporting sprockets 45 and 65, and a plurality of holes 21 being formed in the frame body 20, a chain separator 50 fixed to the frame body 20 and configured to guide movement of a chain member for left-right bending 40 and a chain member for up-down bending 60 forward and backward, and a first chain cover 30 sandwiched between the frame body 20 and the chain separator 50, on a first surface 30i of the first chain cover 30, a plurality of projecting sections fitting in the plurality of holes 21 being formed.
