A multi-plate oil cooler with high joining strength with an inner fin while suppressing a usage amount of a nickel brazing filler, even when ferrite-based stainless steel with low wettability for a nickel brazing filler is used for a plate is provided. A plate made from a ferrite-based stainless steel plate and an inner fin made from a pure iron plate or a carbon steel plate are joined with a nickel brazing filler to form a multi-plate oil cooler. In general, a structure body is provided wherein a ferrite-based stainless steel and pure iron or a carbon steel are joined with a nickel brazing filler.