To provide a waterproof sheet which can be constructed in a short period of time without impairing water tightness and suppress a construction cost, and to provide a method for installing the waterproof sheet.

The method allows a waterproof sheet 1 to be installed in a short period of time because the method bonds the waterproof sheet 1 to an object at an adhesive part 6 without heating a waterproof sheet body 2. When the waterproof sheets 1 are bonded, the method heats and melts a surface side modified asphalt layer 22 in a surface side bonding region 22a and a rear face side modified asphalt layer 23 in a rear face side bonding region 5a to bond them to each other at a melted portion. Accordingly, both of the waterproof sheets 1 are closely bonded and can secure water tightness between themselves.