To provide a method for manufacturing a round corner R flat square rolled material as hot-rolled by the caliber-less flat square rolling by using a round bar stock in a small-amount and size-free manner.

In the method for manufacturing the corner R flat square rolled material to provide roundness at the corners of a flat square rolled material, a round bar stock 2 is subjected to pass-rolling five times, i.e., a first horizontal rolling H1, a first vertical rolling V1, a second horizontal rolling H2, a second vertical rolling V2, and a third horizontal rolling H3 by using a caliber-less rolling mill 1, and a flat square rolled material 7 with a round corner R is manufactured thereby.