An adapter comprising: an optical connector accommodation part having: a first cavity in which a first optical connector having a first front end portion is to be accommodated, and a second cavity in which a second optical connector having a second front end portion is 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; a first spacer part having the first surface; a second spacer part having the second surface; a spacer elastic member (67c, 67d) configured to elastically couple the first spacer part and the second spacer part; and a light transmission part configured to enable a light beam, which propagates between a first optical interface part (IF-1) provided at the first front end portion and a second optical interface part (IF-2) provided at the second front end portion, to pass therethrough, and the spacer being arranged between the first cavity and the second cavity, wherein at a state where the first front end portion is contacted to the first surface and the second front end portion is contacted to the second surface, the first optical interface part (IF-1) and the second optical interface part (IF-2) face each other at a predetermined interval, wherein the optical connector accommodation part has a third cavity formed between the first cavity and the second cavity, wherein the spacer is accommodated in the third cavity, wherein at a first state before the first optical connector and the second optical connector are positioned with respect to each other through the spacer, the first surface and second surface of the spacer are contacted to inner wall surfaces (26a, 26b) defining the third cavity, wherein at a second state after the first optical connector and the second optical connector are positioned with respect to each other through the spacer, the first surface and second surface of the spacer are spaced from the inner wall surfaces, wherein the spacer further has a guide pin, and wherein at a state where the guide pin is inserted into a guide hole formed at the first optical connector and a guide hole formed at the second optical connector, the first optical connector and the second optical connector are positioned with respect to each other. an optical connector accommodation part having: a first cavity in which a first optical connector having a first front end portion is to be accommodated, and a second cavity in which a second optical connector having a second front end portion is to be accommodated; and a first cavity in which a first optical connector having a first front end portion is to be accommodated, and a second cavity in which a second optical connector having a second front end portion is 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; a first spacer part having the first surface; a second spacer part having the second surface; a spacer elastic member (67c, 67d) configured to elastically couple the first spacer part and the second spacer part; and a light transmission part configured to enable a light beam, which propagates between a first optical interface part (IF-1) provided at the first front end portion and a second optical interface part (IF-2) provided at the second front end portion, to pass therethrough, and 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; a first spacer part having the first surface; a second spacer part having the second surface; a spacer elastic member (67c, 67d) configured to elastically couple the first spacer part and the second spacer part; and a light transmission part configured to enable a light beam, which propagates between a first optical interface part (IF-1) provided at the first front end portion and a second optical interface part (IF-2) provided at the second front end portion, to pass therethrough, and the spacer being arranged between the first cavity and the second cavity, wherein at a state where the first front end portion is contacted to the first surface and the second front end portion is contacted to the second surface, the first optical interface part (IF-1) and the second optical interface part (IF-2) face each other at a predetermined interval, wherein the optical connector accommodation part has a third cavity formed between the first cavity and the second cavity, wherein the spacer is accommodated in the third cavity, wherein at a first state before the first optical connector and the second optical connector are positioned with respect to each other through the spacer, the first surface and second surface of the spacer are contacted to inner wall surfaces (26a, 26b) defining the third cavity, wherein at a second state after the first optical connector and the second optical connector are positioned with respect to each other through the spacer, the first surface and second surface of the spacer are spaced from the inner wall surfaces, wherein the spacer further has a guide pin, and wherein at a state where the guide pin is inserted into a guide hole formed at the first optical connector and a guide hole formed at the second optical connector, the first optical connector and the second optical connector are positioned with respect to each other.