A receptacle connector comprising: a receptacle ferrule having: a first holding part configured to hold an end portion of a first optical fiber, and a first front end portion having a receptacle interface part (IF-1) configured to be optically coupled with the first optical fiber; a receptacle housing in which the receptacle ferrule and a plug connector having a second front end portion having a plug interface part (IF-2) configured to be optically coupled with the receptacle interface part are to be accommodated; and a spacer having: a first surface configured to contact the first front end portion; a second surface positioned at an opposite side to the first surface and configured to contact the second front end portion, and a light transmission part configured to enable a light beam propagating between the receptacle interface part and the plug interface part to pass therethrough, the spacer being arranged in the receptacle housing, wherein at a state where the first front end portion is contacted to the first surface of the spacer and the second front end portion is contacted to the second surface of the spacer, the receptacle interface part and the plug interface part face each other at a predetermined interval, characterized in that at a first state where the receptacle interface part and the plug interface part are not positioned with respect to each other via the spacer, the second surface of the spacer is contacted to an inner wall surface (36a, 36b) defining a cavity of the receptacle housing, wherein at a second state where the receptacle interface part and the plug interface part are positioned with respect to each other via the spacer, the second surface of the spacer is spaced from the inner wall surface, wherein the receptacle connector further comprises: a support member arranged in the receptacle housing so as to support the receptacle ferrule, and configured to restrain movement of the receptacle ferrule in an insertion direction in which the plug connector is to be inserted into the receptacle housing; and an elastic member arranged between the receptacle ferrule and the support member and configured to be elastically deformed in the insertion direction so that the second surface of the spacer is spaced from the inner wall surface. a receptacle ferrule having: a first holding part configured to hold an end portion of a first optical fiber, and a first front end portion having a receptacle interface part (IF-1) configured to be optically coupled with the first optical fiber; a first holding part configured to hold an end portion of a first optical fiber, and a first front end portion having a receptacle interface part (IF-1) configured to be optically coupled with the first optical fiber; a receptacle housing in which the receptacle ferrule and a plug connector having a second front end portion having a plug interface part (IF-2) configured to be optically coupled with the receptacle interface part are to be accommodated; and a spacer having: a first surface configured to contact the first front end portion; a second surface positioned at an opposite side to the first surface and configured to contact the second front end portion, and a light transmission part configured to enable a light beam propagating between the receptacle interface part and the plug interface part to pass therethrough, a first surface configured to contact the first front end portion; a second surface positioned at an opposite side to the first surface and configured to contact the second front end portion, and a light transmission part configured to enable a light beam propagating between the receptacle interface part and the plug interface part to pass therethrough, the spacer being arranged in the receptacle housing, wherein at a state where the first front end portion is contacted to the first surface of the spacer and the second front end portion is contacted to the second surface of the spacer, the receptacle interface part and the plug interface part face each other at a predetermined interval, characterized in that at a first state where the receptacle interface part and the plug interface part are not positioned with respect to each other via the spacer, the second surface of the spacer is contacted to an inner wall surface (36a, 36b) defining a cavity of the receptacle housing, wherein at a second state where the receptacle interface part and the plug interface part are positioned with respect to each other via the spacer, the second surface of the spacer is spaced from the inner wall surface, wherein the receptacle connector further comprises: a support member arranged in the receptacle housing so as to support the receptacle ferrule, and configured to restrain movement of the receptacle ferrule in an insertion direction in which the plug connector is to be inserted into the receptacle housing; and an elastic member arranged between the receptacle ferrule and the support member and configured to be elastically deformed in the insertion direction so that the second surface of the spacer is spaced from the inner wall surface. a support member arranged in the receptacle housing so as to support the receptacle ferrule, and configured to restrain movement of the receptacle ferrule in an insertion direction in which the plug connector is to be inserted into the receptacle housing; and an elastic member arranged between the receptacle ferrule and the support member and configured to be elastically deformed in the insertion direction so that the second surface of the spacer is spaced from the inner wall surface.