To provide a positioning ruler for a joint piece for a waterproof sheet, which enables the accurate positioning of a joint piece without irrespective of the positioning accuracy of the waterproof sheet, which can be applied irrespective of the laying order of the waterproof sheet and the joint piece (holding plate), and which enables a marking operation to be omitted in low cost.

This ruler is composed of a flexible sheet with a thickness corresponding to the thickness of the joint piece. In the ruler, guide portions 4a-4c corresponding to an outer shape of the joint piece are pierced in positions corresponding to arrangement patterns of the joint piece.