The present invention employs a polyimide film, which has a coefficient of thermal expansion (CTE) that is a negative number at a temperature equal to or greater than 350° C., as a debonding layer for separating a flexible substrate and a carrier substrate, and thus can easily separate a flexible substrate from a carrier substrate by using a detaching phenomenon caused by a difference in residual stress between the flexible substrate and the debonding layer after a high-temperature process for producing an element on the flexible substrate. Therefore, the present invention can separate the flexible substrate without causing chemical or physical damage to the element formed on the flexible substrate, thereby minimizing problems that may occur during a stripping process.