To provide a waterproof sheet laying method for easily and correctly abutting the center of a high-frequency oscillating head of a high-frequency heating device on a central portion of a fixed member via a waterproof sheet, and to provide a connection position determining jig for use in the method.

There is provided a connection position determining jig having a guide hole for determining a connection position. The guide hole is formed in a midpoint of a nonconductive plate having a thickness almost corresponding to a fixed member which is for use in laying work of the waterproof sheet, and has a size larger than the rim of the fixed member and smaller than the rim of a high-frequency oscillating head. The thus formed jig is mounted on the fixed member via the waterproof sheet so that the guide hole corresponds to the fixed member, and thereafter the high-frequency oscillating head is abutted on the guide hole, whereby the fixed member is connected to the waterproof sheet by high-frequency induction heating.