when the plug 11 is inserted into the socket 21 mechanical stress is not generated between each of the plug contacts 13 and the corresponding one of the socket contacts 22 subsequently as shown in fig3 c the locking member 14 has been inserted between the pair of connecting members 12 via the through hole 32 of the plug 11 at this time side faces of the locking member 14 abut respectively on the inner side faces 12 b each of which is inclined of the connecting members 12 and thereby press the inner side faces 12 b outward accordingly the pair of connecting members 12 is expanded outward respectively in the right and left directions as described above the inclination angle u03b8 1 of each of the inner face sides 12 b of the connecting members 12 has substantially the same absolute value as that of the inclination angle u03b8 2 of each of the inner side faces 21 a of the recess portion 23 in addition each of the outer side faces 12 a of the connecting members 12 is formed approximately perpendicular to the printed wiring boards 31 and 41 for this reason the outer side faces 12 a of the connecting members 12 are brought into contact respectively with the inner side faces 21 a of the recess portion 23 in a state where the locking member 14 is fully inserted.