The operability and usage durability of a lock cancellation operating part (12e) can be improved with a simple configuration. A latch lock part (12a) and a lock cancellation operating part (12e) are integrally provided to an elastically-displaceable lock arm member (12g), which is integrally extending like a cantilever from a board connecting part (12c) solder-joined with a main wiring board. The board connecting part (12c), the latch lock part (12a), and the lock cancellation operating part (12e) are integrally provided with the lock arm member (12g) so that a retaining action and a cancelling action of the inserted signal transmission medium (F) is carried out by operation of a single member.