A locking structure LS includes: a locking protrusion provided on a front holder and formed to protrude in a vertical direction; locking arms provided on a female housing and formed to extend in a fitting direction; and regulating surfaces provided to face the locking protrusion in a width direction and regulate movement of the locking arms in the width direction. The locking arms are inserted between the locking protrusion and the regulating surfaces, have locking portions formed to partially overlap with the locking protrusions, and can be elastically deformed in the vertical direction with respect to the female housing.