To provide a cross-linked material made of a polylactic acid having approximately 100% elongation at break required for being used as an usual plastic at ≤60°C which is a transition temperature of a polylactic acid, and capable of maintaining its shape even on becoming a high temperature of ≥60°C.

This method for producing the cross-linked material made of the lactic acid is provided by mixing a plasticizer containing at least a rosin derivative and a cross-linking monomer with the polylactic acid, kneading, forming the kneaded material as a prescribed form and irradiating the formed material with an ionizing radiation for cross-linking. The irradiated dose of the ionizing radiation is made as ≥10 and ≤100 kGy, and the plasticizer is blended by 15-30 wt% based on 100 wt% polylactic acid.