axis 31 in the flange 47 fig3 of the ferrule 15 on the other hand the key 23 is formed on the stopper wall 51 fig5 so as to extend along the axis 31 toward the rear side of the housing 19 and the key width gradually decreases along the axis 31 toward the tip more specifically the key 23 is formed in a shape including base portion 23 a which has a width d 1 substantially corresponding with the groove width w of the key groove 21 front portion 23 b whose width d 2 is narrower than the base portion 23 a and which is connected with the base portion 23 a via taper portion t and front chamfered portion 53 fig5 in the optical connector 17 the rotation and headshake of the ferrule 15 are controlled with high precision by engagement of the key groove 21 with the base portion 23 a of the key 23 until the ferrule 15 accomplish pc coupling with a counterpart connector when the ferrule 15 retreats by a given distance b because of contraction of the elastic means upon pc coupling the rotation of the ferrule 15 is regulated by the clearance between the base portion 23 a and the key groove 21 area b.