A joint structure comprises a light-absorbable member having at least one opening portion and a light-permeable member superposed on the light-absorbable member so as to cover the opening portion, wherein an annular weld part is formed so as to enclose the opening portion and join the light-absorbable member and the light-permeable member, and the light-permeable member is formed into a thin sheet adhering to the light-absorbable member by deforming at a depressurized state of an interior of the opening portion before the formation of the annular weld part.

A method for preparing a resizing sleeve for lining a joining point in a pipe assembly having pipes of thicker and thinner diameter. The method includes stretching the resizing sleeve in diameter to the size of the thicker pipe in diameter of the pipe assembly, installing an installation hat according to the thicker pipe in diameter of the pipe assembly by airtightly gluing it onto the stretched resizing sleeve, and returning the resizing sleeve to the size of the thinner pipe in diameter of the pipe assembly to be lined. A suitable tool for implementing the method is also disclosed.