The present invention relates to a waterproofing method for an outdoor tank, the waterproofing method being characterized in that the boundary portion between the bottom of the outdoor tank installed on a foundation and the foundation is covered in a liquid tight manner by a waterproof sheet adhered so as to extend from the outdoor tank to the foundation. The present invention can provide a waterproofing method which is effective to prevent the entry of rain water into the boundary portions between the bottoms of outdoor tanks and the foundations, most of the outdoor tanks being large tanks mainly installed in oil refineries or the like. This waterproofing method for an outdoor tank can more reliably waterproof the bottom side of the outdoor tank using the simple method.