In a manufacturing method of an assembly, the assembly including a metal part and a pipe, the pipe including a material containing a resin, an adhesive is first adhered to an outer circumferential surface of the pipe and a metal part covering at least a portion of an outer circumferential surface of the pipe. Here, an area to which the adhesive is adhered on the outer circumferential surface of the pipe is defined as an adhesion area. Next, by a heat source provided inside the pipe, a target area is heated without interposing the metal part. The target area is located radially interior to the adhesion area and located on an inner circumferential surface of the pipe.