An optical system which can be detached and attached is constituted such that it is arranged at the object side of an objective optical system for endoscope in which an incidence pupil is arranged, and a middle image of the objective is formed at its inside, and in the arrangement state, the incidence pupil of the objective optical system for endoscope is relayed at a predetermined position which is more projected toward the utmost object side than a surface at the object side in the endoscope. (An incidence pupil which is conjugate with an incidence pupil is arranged at a position which is more projected toward the object side by a predetermined amount than a surface which is at the utmost object side in the endoscope).
