To provide a structure of a forceps plug attaching part of an endoscope for stably operating an injection cylinder without a forceps plug being significantly wobbled even in the case of performing a liquid chemical injecting operation by inserting the injection cylinder to the forceps plug.

First and second circumferential grooves 32A and 32B are parallelly formed on the outer peripheral surface of a part projected from the surface of an operation part 2 of a treating tool insertion base 30. Inside a recessed part 19 of the forceps plug 10, first and second small diameter parts 13A and 13B to be fitted to the first and second circumferential grooves 32A and 32B when the treating tool insertion base 30 is fitted are projectingly formed from an inner peripheral wall to the inner part. While the treating tool insertion base 30 is fitted to the recessed part 19 of the forceps plug 10, the state that the first and second recessed parts 19 respectively tighten the bottom surfaces of the first and second circumferential grooves 32A and 32B is attained.