PURPOSE: To enable mass-production of thin sheet joined products of radiator, etc., by using casting method and joining plural thin sheet bodies of extremely thin sheet state having projecting length required for the function in some degree as projecting condition.

CONSTITUTION: By using a supporting tool 3 freely releasable from a mold into the mold 2 for casting joining base body 1, one end part 4 of plural thin sheet bodies 4 to be joined as the projecting condition, are set to the joining base body 1 at the prescribed interval. Molten metal is poured into thin mold 2 to cast the joining base body 1 and the thin sheet joined product A of radiator, etc., produced by projecting plural thin sheet bodies 4 in the joining base body 1 is cast.