A waterproofing method is provided for a wire to be arranged in a water susceptible area of a vehicle. An insulating coating layer is removed in a lengthwise intermediate part of the wire to expose a core. A heat shrinkable tube with an inner layer made of a hot-melt waterproofing agent is mounted on the exposed core section and parts of the insulating coating layer adjacent to the exposed core section and heated. As a result, the hot-melt waterproofing agent of the inner layer is melted and infiltrates into clearances between strands of the exposed core section. In addition, negative pressure is introduced into the inside of the insulating coating layer from an end of the wire, thereby sucking the waterproofing agent into the inside of the insulating coating layer to infiltrate the waterproofing agent also between the strands of the core inside the insulating coating layer.