A housing of a lower connector has an opening on a side of an upper connector, and houses two movable parts movably from a first state of, by having upper portions thereof engaged with the opening, being in proximity to each other to form a guide hole, and a second state of, by releasing the engagement, being distanced from an external signal terminal. At positions contactable with the two movable parts, the housing has a protrusion whose tip gradually tapers toward the two movable parts in directions in which the two movable parts face each other. The protrusion guides the two movable parts in mutually separating directions, when the engagement between the opening of the housing and the upper portions of the two movable parts is released associated with depression of the upper connector, and the tip enters the gap through which the two movable parts face each other.