A waterproofing method is provided for a wire to be arranged in a water susceptible area of a vehicle. The method includes removing an insulating coating layer in a lengthwise intermediate part of the wire to expose a core. A fluid waterproofing agent then is dropped to an exposed core section. Negative pressure then is introduced into the inside of the insulating coating layer from wire ends to suck the waterproofing agent into the inside of the insulating coating layer. Alternatively the exposed core section is loaded with pressure air to press the waterproofing agent into the inside of the insulating coating layer. Thus the waterproofing agent is infiltrated between the strands inside the insulating coating layer.