A water cutoff structure of a covered wire comprising an upper side member and a lower side member for carrying out water cutoff to the covered wire by sandwiching a covered wire constituted by covering core lines by a pair of water cutoff members made of a resin respectively provided at the upper side member and the lower side member to be subjected to ultrasonic welding, in which the lower side member of the water cutoff structure is provided with a pair of locking portions and the pair of locking portions are respectively fitted to a pair of locking portions provided to correspond to the upper side member. Ultrasonic vibration energy is transmitted to concentrate on a welded portion.